Set rabbitmq loopback_users explicitely

Set rabbitmq loopback_users explicitely

In Fedora/RHEL land we carry a patch that sets the loopback_users
config explicitely to []. Since this patch diverges from upstream
and sometimes gets dropped by mistake during rebases, let's set
this value explicitely in our config files, instead of relying
on a patch that is distro-specific.

The patch is here:
http://pkgs.fedoraproject.org/cgit/rpms/rabbitmq-server.git/tree/rabbitmq-server-0004-Allow-guest-login-from-non-loopback-connections.patch

Change-Id: If9ca05b38a8bd2a6834c08336a816bbd0ae1ea94