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.
actually...not sure caching this is a good idea. if someone adds more vhosts they would have to know to restart the agent which seems a bit odd. What do you think?