Add plug-in summary for osc doc

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

Change-Id: I0174d31c9fbedb2a59aa41caac268bda8df8b0da

saharaclient/osc/plugin.py

@@ -13,6 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+"""OpenStackClient plugin for Data Processing service."""
+
 from osc_lib import utils
 from oslo_log import log as logging