Revert back to 'call' for agent reports

Fixes bug 1159581

The logic of the agents is based on RPC call()
since the agent needs to catch an exception over RPC.

Change-Id: I6d408216ebdfd130bab2555e61febd02b521985d

quantum/agent/rpc.py

@@ -58,7 +58,7 @@ def __init__(self, topic):
             topic=topic, default_version=self.BASE_RPC_API_VERSION)
 
     def report_state(self, context, agent_state):
-        return self.cast(context,
+        return self.call(context,
                          self.make_msg('report_state',
                                        agent_state={'agent_state':
                                                     agent_state},