Add a target to Sinkhole so it's compatible

This prevents oslo messaging dispatch from trying
to treat the receiver function as a target. When it
thought it was a target it was referencing attributes
it didn't have.

Closes-Bug: #1710064
Change-Id: I1832a05f23cd76c1732d3b98539988bc9519d79e
(cherry picked from commit e0a2602904)

neutron/plugins/ml2/drivers/openvswitch/agent/ovs_neutron_agent.py

@@ -384,6 +384,8 @@ class OVSNeutronAgent(l2population_rpc.L2populationRpcCallBackTunnelMixin,
 
     def setup_old_topic_sinkhole(self):
         class SinkHole(object):
+            target = oslo_messaging.Target(version='1.4')
+
             def __getattr__(self, attr):
                 return self._receiver