Update executor to eventlet

Default 'blocking' executor handles only one message at once.
'eventlet' executor is recommended.

Change-Id: Id738d0462fbb3c7fd6c78ee2f0dd0f1e79131ca7
Closes-Bug: #1517843

watcher/tests/common/messaging/test_messaging_handler.py

@@ -58,6 +58,7 @@ class TestMessagingHandler(base.TestCase):
             handler.transport,
             m_target,
             [self.ENDPOINT],
+            executor='evenlet',
             serializer=None,
         )