Set deployment backends per environments

This removes the "global" setting of deployment backend and relies that the different sub-types of DNEnvironment will set this in the $deploymentBackend variable.
Look at how we do deprecation in the sqlite module.

https://github.com/silverstripe-labs/silverstripe-sqlite3/blob/master/_config.php
and
https://github.com/silverstripe-labs/silverstripe-sqlite3/blob/master/code/SQLite3Database.php#L79