Fix and Use rabbitmq-server element in boot-stack

Rabbit MQ is a separate service and has configuration logic, so the
entangled configuration within boot-stack was inappropriate.

* Remove rabitmq-server restart in boot-stack firstboot.
* Move custom upstart script from boot-stack into rabbitmq-server
  element.
* Make rabbitmq-server orc properly handle empty heat_users.json file.

Fix bug 1166838

Change-Id: I8985d8020f94c257d426065cf914213b35ec959f