Bifrost no longer deploys RabbitMQ, so we should not try to stop it
during upgrade. In fact, if we do then it fails:

Failed to stop rabbitmq-server.service: Unit rabbitmq-server.service not
loaded.

Bifrost removed RabbitMQ in Train, so this is only required from Ussuri.

Change-Id: Ie86f85974fd7385e72a918065fc9c5172f9684ba