Add plug-in summary for osc doc

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http://docs.openstack.org/developer/python-openstackclient/plugin-commands.html

Change-Id: I06db0b2c04e5e4bb395b68c1640bec7c6588895b

senlinclient/plugin.py

@@ -10,6 +10,8 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+"""OpenStackClient plugin for Clustering service."""
+
 import logging
 
 from openstack import connection