rabbitmq module: modernize after package upgrade

- Use socket-activated epmd - that way there won't be any trouble when
  more than one erlang system is used within a single host.
- Use new automation-friendly configuration file format
- Use systemd notifications instead of buggy 'rabbitmqctl wait' for
  confirming successful server startup.
  'wait' bug: https://github.com/rabbitmq/rabbitmq-server/issues/463
- Use 'rabbitmqctl shutdown' instead of 'stop', because it's not
  pid-file based
- Use sane systemd unit defaults from RabbitMQ repo:
  https://github.com/rabbitmq/rabbitmq-server/blob/master/docs/rabbitmq-server.service.example
- Support for external plugins