Add update-snapshot test

* change method test_snapshot_create_get_list_delete for VolumesSnapshotTest
* add method update_snapshot for SnapshotsClientJSON & SnapshotsClientXML

Change-Id: I09091b5859d3b5c73042efba0ecbfd96b4f5c0ea
diff --git a/tempest/services/volume/json/snapshots_client.py b/tempest/services/volume/json/snapshots_client.py
index ce2da90..10ba3fd 100644
--- a/tempest/services/volume/json/snapshots_client.py
+++ b/tempest/services/volume/json/snapshots_client.py
@@ -76,6 +76,14 @@
         body = json.loads(body)
         return resp, body['snapshot']
 
+    def update_snapshot(self, snapshot_id, **kwargs):
+        """Updates a snapshot."""
+        put_body = json.dumps({'snapshot': kwargs})
+        resp, body = self.put('snapshots/%s' % snapshot_id, put_body,
+                              self.headers)
+        body = json.loads(body)
+        return resp, body['snapshot']
+
     # NOTE(afazekas): just for the wait function
     def _get_snapshot_status(self, snapshot_id):
         resp, body = self.get_snapshot(snapshot_id)