Removed use of deprecated LOG.warn method

LOG.warn is deprecated. We were still used it some places.
So I replaced the LOG.warn method to LOG.warning, which is not
deprecated.

Change-Id: I9461cec569445ad6c40db9ce2feeeba1ef0af0e3
Closes-Bug: #1508442

watcher/common/messaging/messaging_handler.py

@@ -94,7 +94,7 @@ class MessagingHandler(threading.Thread):
                 )
                 self.__server = self.build_server(target)
             else:
-                LOG.warn(
+                LOG.warning(
                     _LW("No endpoint defined; can only publish events"))
         except Exception as e:
             LOG.exception(e)