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 think it's ok to be redundant. 
The warning would be displayed in the info page and the infrastructure overview, so i think it does make sense.