CRM-20932 - CRM_Utils_System_Joomla - Fix warning. Define JDEBUG

CRM-20932 - CRM_Utils_System_Joomla - Fix warning. Define JDEBUG

Background
==========

When running `cv en <extname>` on Joomla, there are repeated warnings, "Use
of undefined constant JDEBUG", and each includes a backtrace.  This is hard
to read.

The bug appears to affect backend scripts; it's discussed with a solution in:

https://github.com/joomla/joomla-cms/issues/11512#issuecomment-244937966

Before
======

`CRM_Utils_System_Joomla::loadBootStrap()` loads various config files.
However, these may or may not set `JDEBUG`.

After
=====

`CRM_Utils_System_Joomla::loadBootStrap()` defensively sets `JDEBUG` if no
other part of the bootstrap has done so.