Fixed reddwarf connection url (db_url) to get set correctly.

Devstack commit afc29fe5f210a8e78bafaafe7e8b3b5de733971c (Deprecate
DATABASE_TYPE and use_database) broke us such that the reddwarf
connection url was not being set correctly.

The Database type is now selected by adding it to ENABLED_SERVICES like
any other service. This fix makes sure that the ENABLED_SERVICES are set
before lib/database is sourced.

Fixes bug: 1131935

Change-Id: I7a122676e9fdf569fde3d3e57b81925d41e48b20