Fix issues introduced /w migration to Credentials

Managers and base classes were refactored to use Credentials
but this change missed an issue that slipped through the gate:

* The Baremetal scenario base was not updated, causing breakage
  in the Ironic gate.  This ensures BaremetalScenarioTest base class
  properly instantiates its baremetal_client using the correct Credentials
  object.

Change-Id: I5577edcd8da14275eb336ee8137eade2084756c9
Merge "Fix issues introduced /w migration to Credentials"