Add a service check to the RabbitMQ check based on aliveness.

Changes:
- Adds optional configuration to have a list of vhosts you want to
  monitor. The default case will fetch all vhosts from the API.
- Adds `rabbitmq.aliveness` check, tagged by vhost.
I'm not sure it's a warning case. The response when rabbitMQ is down would get to this line so adding a self.warning would be redundant because we're already reporting a critical status. The failure response for this check is a bad status code and/or some response that can't be parsed as JSON.