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 
(Supporting MSSQL by default and then not having schema updates supported and just state "Contact support@owncloud.com" is not really a very good UX too)