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 tested: "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".
code cleanup during review :+1:
Merge pull request #11786 from owncloud/MakeSupportedDBsConfigurable

Make supported DBs configurable within config.php