Fix nova deployment failure when rabbitmq is disabled

Nova always tries to create the rabbitmq user regardless of
whether RabbitMQ is enabled or not.
This ps also adds an external rabbitmq doc.

Change-Id: Iec517226e4c82ea351889b55689a3efceaadcc76
(cherry picked from commit 24d0814)

Author: wchy1001

ansible/roles/nova-cell/tasks/rabbitmq.yml

@@ -24,4 +24,6 @@
         - nova_cell_rpc_group_name != nova_cell_notify_group_name or
           nova_cell_rpc_rabbitmq_users != nova_cell_notify_rabbitmq_users
 
-  when: nova_cell_rpc_transport == 'rabbit'
+  when:
+    - nova_cell_rpc_transport == 'rabbit'
+    - enable_rabbitmq | bool

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

@@ -0,0 +1,53 @@
+.. _external-rabbitmq-guide:
+
+=================
+External RabbitMQ
+=================
+
+Sometimes, for various reasons (Redundancy, organisational policies, etc.),
+it might be necessary to use an external RabbitMQ cluster.
+This use case can be achieved with the following steps:
+
+Requirements
+~~~~~~~~~~~~
+
+* An existing RabbitMQ cluster, reachable from all of your
+  nodes.
+
+Enabling External RabbitMQ support
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In order to enable external RabbitMQ support,
+you will first need to disable RabbitMQ deployment,
+by ensuring the following line exists within ``/etc/kolla/globals.yml`` :
+
+.. code-block:: yaml
+
+   enable_rabbitmq: "no"
+
+
+Overwriting transport_url within ``globals.yml``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When you use an external RabbitMQ cluster, you must overwrite
+``*_transport_url`` within ``/etc/kolla/globals.yml``
+
+.. code-block:: yaml
+
+   rpc_transport_url:
+   notify_transport_url:
+   nova_cell_rpc_transport_url:
+   nova_cell_notify_transport_url:
+
+For example:
+
+.. code-block:: yaml
+
+   rpc_transport_url: rabbit://openstack:[email protected]:5672,openstack:[email protected]:5672,openstack:[email protected]:5672//
+   notify_transport_url: "{{ rpc_transport_url }}"
+   nova_cell_rpc_transport_url: rabbit://openstack:[email protected]:5672//
+   nova_cell_notify_transport_url: "{{ nova_cell_rpc_transport_url }}"
+
+.. note::
+
+   Ensure the rabbitmq user used in ``*_transport_url`` exists.

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

@@ -8,3 +8,4 @@ This section describes configuration of message queue services.
    :maxdepth: 1
 
    rabbitmq
+   external-rabbitmq-guide