Check if plugin supports starting rpc listeners

When neutron starts an rpc worker, it checks if the plugin has the method
"start_rpc_listeners".  Since most plugins inherit from a base class, and
that base class implements the start_rpc_listeners method and raises
NotImplementedError, the rpc worker will attempt to call that method.
It should just catch the NotImplementedError and continue on.

Change-Id: Ie1830b6140acffffd0f283a0d8eefa52067f7650
Closes-Bug: 1551542

neutron/service.py

@@ -139,7 +139,10 @@ class RpcWorker(worker.NeutronWorker):
         super(RpcWorker, self).start()
         for plugin in self._plugins:
             if hasattr(plugin, self.start_listeners_method):
-                servers = getattr(plugin, self.start_listeners_method)()
+                try:
+                    servers = getattr(plugin, self.start_listeners_method)()
+                except NotImplementedError:
+                    continue
                 self._servers.extend(servers)
 
     def wait(self):