Add plug-in summary for osc doc

Stevedore Sphinx extension handles this comment.
http://docs.openstack.org/developer/python-openstackclient/plugin-commands.html

Change-Id: I57cf816c30fadc021214725c5018363e66d5c741

zaqarclient/queues/cli.py

@@ -12,6 +12,8 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+"""OpenStackClient plugin for Messaging service."""
+
 import logging
 
 from osc_lib import utils