Messaging backend selection

There is another patch that removes rabbitmq-server from
boot-stack's element dependencies.

https://review.openstack.org/#/c/72516/

This patch should be deployed before 72516.

This patch selects rabbitmq-server as the default messaging
backend using the *EXTRA_ARGS environment variable.

Change-Id: Ic19af7f6071af04bd6bbce8196bf80ab2dae6840
Merge "Messaging backend selection"