Add image signing scenario

This change adds the first scenario test to the
Barbican Tempest plugin. This scenatio tests Nova
and Glance's image signature verification
functionality.

Depends-On: Ifdf8b426c21e4b3a51f97cbc3d95eb842eb04515
Change-Id: Id9629ecbbc75e19eec81f60daec7b0a085bcdc12
diff --git a/barbican_tempest_plugin/clients.py b/barbican_tempest_plugin/clients.py
index 078f383..9c009ba 100644
--- a/barbican_tempest_plugin/clients.py
+++ b/barbican_tempest_plugin/clients.py
@@ -12,14 +12,22 @@
 # License for the specific language governing permissions and limitations under
 # the License.
 
+from tempest import clients
+from tempest.common import credentials_factory as common_creds
 from tempest import config
-from tempest.lib.services import clients
-
+from tempest.lib.services import clients as cli
 
 CONF = config.CONF
 
+ADMIN_CREDS = common_creds.get_configured_admin_credentials()
 
-class Clients(clients.ServiceClients):
+
+class Manager(clients.Manager):
+    def __init__(self, credentials=ADMIN_CREDS):
+        super(Manager, self).__init__(credentials)
+
+
+class Clients(cli.ServiceClients):
     """Tempest stable service clients and loaded plugins service clients"""
 
     def __init__(self, credentials, service=None):