CRM-20768 - install - Accept mysql port

CRM-20768 - install - Accept mysql port

== Background ==

In previous releases, the web-based installation script allowed you to enter
the MySQL "server" with a `host:port` combination (e.g.  `127.0.0.1:3307`).

I believe this regressed during the mysql=>mysqli transition.

== Before ==

If you use the web-based install UI and enter a `host:port` combination, the
error messages indicate a confusing mix of information (e.g.  some
indicating port 3306, some indicating port 3307).

== After ==

If you use the web-based install UI and enter a `host:port` combination, it
uses the specified port.

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* CRM-20768: Web-based installation should accept non-standard MySQL port
  https://issues.civicrm.org/jira/browse/CRM-20768