Docs: update RabbitMQ HA instructions

``kolla-ansible genconfig`` includes some tasks [1] that require
containers to still be running. Move the config generation to be done
before containers are stopped.

Also make it clearer that containers should not be restarted with the
new config until after RabbitMQ state has been reset.

1. https://github.com/openstack/kolla-ansible/blob/master/ansible/roles/octavia/tasks/get_resources_info.yml#L2

Change-Id: I1d4fc32e5963a8e8fa4a8fc7c467cadf0f26fd5f

Author: MoteHue

doc/source/reference/message-queues/rabbitmq.rst

@@ -126,18 +126,19 @@ different type, the follow procedure will be needed.
    release, following procedure is required to be carried out before any
    upgrade to Epoxy.
 
-1. Stop all OpenStack services which use RabbitMQ, so that they will not
-   attempt to recreate any queues yet.
+1. Generate the new config for all services. After this, make sure not to
+   restart any containers until after the RabbitMQ state has been reset.
 
    .. code-block:: console
 
-      kolla-ansible stop --tags <service-tags>
+      kolla-ansible genconfig
 
-2. Generate the new config for all services.
+2. Stop all OpenStack services which use RabbitMQ, so that they will not
+   attempt to recreate any queues yet.
 
    .. code-block:: console
 
-      kolla-ansible genconfig
+      kolla-ansible stop --tags <service-tags>
 
 3. Reconfigure RabbitMQ if you were previously using
    ``om_enable_rabbitmq_high_availability``.