Take an optional rabbit user name as input

Newer versions of rabbitmq (3.3 and later) do not allow the 'guest'
user to access on non-local interfaces.

- Added a new config RABBIT_USERID which defaults to stackrabbit
- Invoked config scripts using that variable

Adopted from:
https://review.openstack.org/#/c/107779/

Change-Id: I43a231c9611b4cc2e390b603aa3bfb49c915bdc5
Closes-Bug: #1343354
Co-Authored-By: Scott Moser <smoser@ubuntu.com>
Merge "Take an optional rabbit user name as input"