Set a correct whitelist for drivers in Ironic

Ironic recently introduced a feature called Drivers
whitelist. It allows to specify the drivers that are
enabled.

The default list is not appropriate for functional and
integration testing so devstack needs to be updated.

Change-Id: Iccc81343f4d5a7dec8b371934a8c9849d9ffac23
Merge "Set a correct whitelist for drivers in Ironic"