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
diff --git a/tempest/scenario/manager.py b/tempest/scenario/manager.py
index c745f4e..0ef34c6 100644
--- a/tempest/scenario/manager.py
+++ b/tempest/scenario/manager.py
@@ -452,8 +452,8 @@
             raise cls.skipException(msg)
 
         # use an admin client manager for baremetal client
-        username, password, tenant = cls.admin_credentials()
-        manager = clients.OfficialClientManager(username, password, tenant)
+        admin_creds = cls.admin_credentials()
+        manager = clients.OfficialClientManager(credentials=admin_creds)
         cls.baremetal_client = manager.baremetal_client
 
         # allow any issues obtaining the node list to raise early