(ops#878) Enforce common signature for CRM_Utils_System::loadBootStrap(). Fix WP...

(ops#878) Enforce common signature for CRM_Utils_System::loadBootStrap(). Fix WP E2E error.

Before
------

1.  Most `CRM_Utils_System_*` classes implement a method `loadBootStrap($params = array(),...)`.  However, `WordPress`
   and `UnitTests` implement a different signature (`loadBootStrap($name = NULL, $pass = NULL)`), and `Soap` does not
   implement it all.  This is problematic -- because the function is invoked in a centralized way (e.g.
   `CRM_Utils_System::loadBootStrap()` invokes `$config->userSystem->loadBootStrap($params, $loadUser, $throwError,
   $realPath)`), which implies that all UF drivers need to support this signature.

2. When running any of the end-to-end (E2E) tests on CiviCRM-WordPress, the output is polluted with these warnings:

```
bknix-dfl:~/bknix/build/wpmaster/wp-content/plugins/civicrm/civicrm] phpunit5 tests/phpunit/E2E/Cache/FastArrayDecoratorTest.php ; echo "exit=$?"
PHPUnit 5.7.27 by Sebastian Bergmann and contributors.

PHP Warning:  strip_tags() expects parameter 1 to be string, array given in /Users/totten/bknix/build/wpmaster/wp-includes/formatting.php on line 4649
PHP Stack trace:
PHP   1. {main}() /Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar:0
PHP   2. PHPUnit_TextUI_Command::main() /Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar:598
PHP   3. PHPUnit_TextUI_Command->run() phar:///Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar/phpunit/TextUI/Command.php:116
PHP   4. PHPUnit_TextUI_TestRunner->doRun() phar:///Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar/phpunit/TextUI/Command.php:186
PHP   5. PHPUnit_Framework_TestSuite->run() phar:///Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar/phpunit/TextUI/TestRunner.php:517
PHP   6. call_user_func:{phar:///Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar/phpunit/Framework/TestSuite.php:679}() phar:///Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar/phpunit/Framework/TestSuite.php:679
PHP   7. E2E_Cache_CacheTestCase::setUpBeforeClass() phar:///Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar/phpunit/Framework/TestSuite.php:679
PHP   8. CRM_Utils_System::loadBootStrap() /Users/totten/bknix/build/wpmaster/wp-content/plugins/civicrm/civicrm/tests/phpunit/E2E/Cache/CacheTestCase.php:43
PHP   9. CRM_Utils_System_WordPress->loadBootStrap() /Users/totten/bknix/build/wpmaster/wp-content/plugins/civicrm/civicrm/CRM/Utils/System.php:1477
PHP  10. wp_authenticate() /Users/totten/bknix/build/wpmaster/wp-content/plugins/civicrm/civicrm/CRM/Utils/System/WordPress.php:495
PHP  11. sanitize_user() /Users/totten/bknix/build/wpmaster/wp-includes/pluggable.php:515
PHP  12. wp_strip_all_tags() /Users/totten/bknix/build/wpmaster/wp-includes/formatting.php:1850
PHP  13. strip_tags() /Users/totten/bknix/build/wpmaster/wp-includes/formatting.php:4649

Warning: strip_tags() expects parameter 1 to be string, array given in /Users/totten/bknix/build/wpmaster/wp-includes/formatting.php on line 4649

Call Stack:
    0.0026     425040   1. {main}() /Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar:0
    0.0766   14246216   2. PHPUnit_TextUI_Command::main() /Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar:598
    0.0766   14246952   3. PHPUnit_TextUI_Command->run() phar:///Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar/phpunit/TextUI/Command.php:116
    0.4267   23405608   4. PHPUnit_TextUI_TestRunner->doRun() phar:///Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar/phpunit/TextUI/Command.php:186
    0.4336   23511184   5. PHPUnit_Framework_TestSuite->run() phar:///Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar/phpunit/TextUI/TestRunner.php:517
    0.4403   23516840   6. call_user_func:{phar:///Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar/phpunit/Framework/TestSuite.php:679}() phar:///Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar/phpunit/Framework/TestSuite.php:679
    0.4403   23516920   7. E2E_Cache_CacheTestCase::setUpBeforeClass() phar:///Users/totten/bknix/civicrm-buildkit/extern/phpunit5/phpunit5.phar/phpunit/Framework/TestSuite.php:679
    0.5065   31975824   8. CRM_Utils_System::loadBootStrap() /Users/totten/bknix/build/wpmaster/wp-content/plugins/civicrm/civicrm/tests/phpunit/E2E/Cache/CacheTestCase.php:43
    0.5065   31976344   9. CRM_Utils_System_WordPress->loadBootStrap() /Users/totten/bknix/build/wpmaster/wp-content/plugins/civicrm/civicrm/CRM/Utils/System.php:1477
    0.6517   57998912  10. wp_authenticate() /Users/totten/bknix/build/wpmaster/wp-content/plugins/civicrm/civicrm/CRM/Utils/System/WordPress.php:495
    0.6517   57998992  11. sanitize_user() /Users/totten/bknix/build/wpmaster/wp-includes/pluggable.php:515
    0.6517   57999088  12. wp_strip_all_tags() /Users/totten/bknix/build/wpmaster/wp-includes/formatting.php:1850
    0.6517   57999736  13. strip_tags() /Users/totten/bknix/build/wpmaster/wp-includes/formatting.php:4649
```

3.  The warning indicates that (on WordPress) the E2E test is not doing what it's trying to do -- specifically,
    `E2E_Cache_CacheTestCase::setUpBeforeClass()` (or, similarly, `CiviEndToEndTestCase::setUpBeforeClass()`) calls
    `CRM_Utils_System::loadBootStrap()` to authenticate as the CMS administrator, but this fails because WP's
    implementation of `loadBootStrap()` does not respect the same signature.

After
-----

1. `CRM_Utils_System_Base` defines an abstract function `loadBootStrap($params = array(),...)`. Any non-compliant driver (which
   uses a different signature or which omits the function) will produce a syntax error.

2. The classes `CRM_Utils_System_{WordPress,UnitTests}` have been updated to comply with the signature.

3. The class `CRM_Utils_System_Soap` nominally complies. However, this class isn't really a UF driver, and it doesn't make sense for
   it to extend this base class, and the implementation is a stub. But we don't need to scope-creep the bugfix.

4. The `loadBootStrap...strip_tags()...` warnings are no longer generated when running E2E tests on WP.

Comments
--------

* I verified that all extant classes in `CRM/Utils/System/*.php` are loadable/compilable (i.e they now comply with the delcared signature).
* The SOAP integration has test-coverage viia `E2E_Extern_SoapTest`.
* In the PR test output, you'll only see E2E results from D7.  *However*, this patch is part of a bigger effort
  (infrastructure/ops#878) which will eventually give some visibility on that.