Instead of using long backend/drivers name, use short name and stevedore
will load plugins for us.
It will prevent this kind of message in logs:
Failed to load 'keystone.catalog.backends.sql.Catalog' using stevedore:
No 'keystone.catalog' driver found,
Also cleanup unit and functional tests that were setting wrong
credential & assignment drivers.
Change-Id: Id3b8ed63ef9a821eba5374af7ed0fd1c8d755e09
1f051ca9b7