Ensure we 'join' on the timer watchdog thread

This object is just another thread and its cancel method
does not join on itself, so we should make sure that we do
to avoid leaving threads lying around.

Change-Id: I37e3170b58627dee93513ec222f9a55001389dad

oslo_concurrency/watchdog.py

@@ -67,3 +67,4 @@ def watch(logger, action, level=logging.DEBUG, after=5.0):
         yield
     finally:
         timer.cancel()
+        timer.join()