Added Secret Metadata client and unit tests

This change adds a REST client and corresponding
unit tests for Barbican's Metadata API resource.

Change-Id: Icd2c6ae1f476b57e4108faada0bda5f1aebc640b
diff --git a/barbican_tempest_plugin/plugin.py b/barbican_tempest_plugin/plugin.py
index 7eee973..68a971a 100644
--- a/barbican_tempest_plugin/plugin.py
+++ b/barbican_tempest_plugin/plugin.py
@@ -44,6 +44,7 @@
             'client_names': [
                 'ConsumerClient',
                 'ContainerClient',
+                'SecretMetadataClient',
                 'SecretClient'
             ],
         }