Set access_policy for messaging's dispatcher
Set access_policy for messaging's dispatcher

oslo.messaging allow dispatcher to restrict endpoint methods
since 5.11.0 in d3a8f280ebd6fd12865fd20c4d772774e39aefa2, set with
DefaultRPCAccessPolicy to fix FutureWarning like:

FutureWarning: The access_policy argument is changing its default
value to <class 'oslo_messaging.rpc.dispatcher.DefaultRPCAccessPolicy'>
in version '?', 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: Ie17832d64075080c17c09df675b216dbe8af16e6

openstack/congress,openstack/congress