Remove rabbitmq-server from boot-stack's element-deps

It is being removed to allow selection of other message brokers.

A parallel patch for tripleo-incubator will select rabbitmq-server
as the default in diskimage-builder image builds.

Change-Id: I530f2e40008d2a2d0dd6510116331f68e018a9be
Merge "Remove rabbitmq-server from boot-stack's element-deps"