The Ubuntu and Debian package installation scripts for
`rabbitmq-server` install `/etc/rabbitmq` (and its contents) owned by
the `rabbitmq` user -- not `root` as Puppet does. This means that
Puppet and `rabbitmq-server` unnecessarily fight over the ownership.
Create the `rabbitmq` user and group, to the same specifications that
the Debian package install scripts do, so that we can properly declare
the ownership of `/etc/rabbitmq`.