Deprecate rpc_backend option

Option "rpc_backend" from group "DEFAULT" is deprecated for removal
(Replaced by [DEFAULT]/transport_url). Its value may be silently
ignored in the future.

Change-Id: Ib9adcd5e9fe058a4780505000ab9fbffe7df638f
Implements: blueprint deprecate-rpc-backend

defaults/main.yml

@@ -54,8 +54,6 @@ keystone_system_shell: /bin/bash
 keystone_system_comment: keystone system user
 keystone_system_user_home: "/var/lib/{{ keystone_system_user_name }}"
 
-keystone_rpc_backend: rabbit
-
 ## Drivers
 keystone_auth_methods: "password,token"
 keystone_identity_driver: sql

releasenotes/notes/remove_rpc_backend-187132a35223d295.yaml

@@ -0,0 +1,5 @@
+---
+deprecations:
+  - |
+    Remove ``keystone_rpc_backend`` option due to deprecation of
+    rpc_backend option in oslo.messaging.

templates/keystone.conf.j2

@@ -20,7 +20,6 @@ log_dir = /var/log/keystone
 
 ## RabbitMQ RPC
 {% if keystone_messaging_enabled | bool %}
-rpc_backend = {{ keystone_rpc_backend }}
 transport_url = rabbit://{% for host in keystone_rabbitmq_servers.split(',') %}{{ keystone_rabbitmq_userid }}:{{ keystone_rabbitmq_password }}@{{ host }}:{{ keystone_rabbitmq_port }}{% if not loop.last %},{% else %}/{{ keystone_rabbitmq_vhost }}{% endif %}{% endfor %}
 
 [oslo_messaging_rabbit]