[install] Update rabbitmq configuration
[install] Update rabbitmq configuration

Update from this commit https://review.openstack.org/#/c/389971/1
in the openstack-manuals project.

Follow-up of I54a104662081ab7b81e200cf6c23f5a8a1e28d61.

Change-Id: Ide7d831a749c394c90ac8303643fd1184bb0c56f
Related-Bug: #1635615

ArchiFleKs/magnum,ArchiFleKs/magnum,openstack/magnum,openstack/magnum