AMQP Channel have to be set espacially to 'watcher'

Default channel used is named 'openstack'. But Watcher AMQP topics are
bound to 'watcher' channel. So, on RCPClient init, we have to set
the channel name to 'watcher' for the target.

Change-Id: Ifdbb187f1ee60f898ef0340e0c037891350ce815
Closes-Bug: #1520599

watcher/decision_engine/rpcapi.py

@@ -58,6 +58,7 @@ class DecisionEngineAPI(MessagingCore):
 
         transport = om.get_transport(CONF, TransportUrlBuilder().url)
         target = om.Target(
+            exchange='watcher',
             topic=CONF.watcher_decision_engine.topic_control,
             version=MessagingCore.API_VERSION)