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/tests/api/base.py b/barbican_tempest_plugin/tests/api/base.py
index c5a7597..58f6bbf 100644
--- a/barbican_tempest_plugin/tests/api/base.py
+++ b/barbican_tempest_plugin/tests/api/base.py
@@ -65,6 +65,9 @@
         cls.container_client = os.secret_v1.ContainerClient(
             service='key-manager'
         )
+        cls.secret_metadata_client = os.secret_v1.SecretMetadataClient(
+            service='key-manager'
+        )
         cls.secret_client = os.secret_v1.SecretClient(service='key-manager')
 
     @classmethod