Without this change using anything other than `rabbit` or the `rabbitmq-env-conf.bat` file will result in `erlang_dist_running_with_unexpected_nodename`
Follow-up to #2673
cc @dumbbell @michaelklishin
Currently RABBITMQ_BASE is always dynamically picked up from the
environment. This change would fix it at the time of configuration
of the service allowing multiple RabbitMQ services to be configured.