Add namespace tags client and tests

As in the doc following, there are namespace tags apis, this patch
is to add them to the code.

[doc]http://developer.openstack.org/api-ref/image/v2/metadefs-index.html

Change-Id: I92b2c58d11828eafd9f876a46581279bb9b26e3d
diff --git a/releasenotes/notes/add-image-clients-tests-49dbc0a0a4281a77.yaml b/releasenotes/notes/add-image-clients-tests-49dbc0a0a4281a77.yaml
index 9d1a003..eaab1f0 100644
--- a/releasenotes/notes/add-image-clients-tests-49dbc0a0a4281a77.yaml
+++ b/releasenotes/notes/add-image-clients-tests-49dbc0a0a4281a77.yaml
@@ -6,4 +6,5 @@
     there are some apis are not included, add them.
 
       * namespace_objects_client(v2)
+      * namespace_tags_client(v2)