Set access_policy for messaging's dispatcher

oslo.messaging allow dispatcher to restrict endpoint methods since
5.11.0 in d3a8f280ebd6fd12865fd20c4d772774e39aefa2, the default
value of access_policy is LegacyRPCAccessPolicy.
set with DefaultRPCAccessPolicy to fix FutureWarning like:

"The access_policy argument is changing its default value to <class
'oslo_messaging.rpc.dispatcher.DefaultRPCAccessPolicy'> in version 'xxx',
please update the code to explicitly set None as the value:
access_policy defaults to LegacyRPCAccessPolicy which exposes private
methods. Explicitly set access_policy to DefaultRPCAccessPolicy or
ExplicitRPCAccessPolicy."

Change-Id: I7a220e6fc27e30dbbde4326c071a3f0390f6da43

sahara/utils/rpc.py

@@ -17,6 +17,7 @@
 from oslo_config import cfg
 from oslo_log import log as logging
 import oslo_messaging as messaging
+from oslo_messaging.rpc import dispatcher
 from oslo_serialization import jsonutils
 
 from sahara import context
@@ -84,11 +85,13 @@ class RPCServer(object):
     def __init__(self, target):
         global MESSAGING_TRANSPORT
 
+        access_policy = dispatcher.DefaultRPCAccessPolicy
         self.__server = messaging.get_rpc_server(
             target=target,
             transport=MESSAGING_TRANSPORT,
             endpoints=[self],
-            executor='eventlet')
+            executor='eventlet',
+            access_policy=access_policy)
 
     def get_service(self):
         return self.__server