Make supported DBs configurable within config.php

This commit will make the supported DBs for installation configurable within config.php. By default the following databases are enabled: "sqlite", "mysql", "pgsql". The reason behind this is that there might be instances where we want to prevent SQLite to be used by mistake.

To test this play around with the new configuration parameter "supportedDatabases".

@DeepDiver1975 @PVince81 @karlitschek @MTRichards 
This feels a bit strange, but mocking is a good reason.
I can't think of any better way.