Fix rabbit_userid for multi node devstack

I43a231c9611b4cc2e390b603aa3bfb49c915bdc5 introduced a new setting
RABBIT_USERID but only set it if rabbit is enabled. In multi node
devstack the second node uses RABBIT_USERID but the service rabbit isn't
enabled on it.

Always set RABBIT_USERID, if a different message queue is used the
variable will just be ignored.

Change-Id: I8568bddda2a5c66235ecae23af58983ee94c720a
Merge "Fix rabbit_userid for multi node devstack"