[ha-guide] use code-block for RabbitMQ installtion
[ha-guide] use code-block for RabbitMQ installtion

The duplicated text "Install RabbitMQ" that becomes markup label
breaks the build of the translated guide.

Change-Id: Ifb8b2fd37f8b13d84f639e170936e67709283d65

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals