Make ping method static.

Without being static or passing a self parameter, the worker will
receive a runtime WAMP error when they attempt to ping during a
health check, this marks them unhealthy every single time you
attempt a health check.

buildman/buildcomponent.py

@@ -274,6 +274,7 @@ class BuildComponent(BaseComponent):
     # Set the component back to a running state.
     self._set_status(ComponentStatus.RUNNING)
 
+  @staticmethod
   def _ping():
     """ Ping pong. """
     return 'pong'