add vhost option

[ghost]

Hello!

Please add rabbitmq vhost selection. My variant in this request.
If this variant is not good - please tell me why

[jasonmcintosh]

Looks pretty good but two changes:

1. The default for vhost should be '%2f' and not 'root' - have to have URL encoded values.
2. Since the vhost argument is ONLY used for aliveness, should be named appropriately, e.g.
   aliveness_vhost instead of just vhost

[jasonmcintosh]

The command "./scripts/rabbitmq/rabbitmq-status.sh check_aliveness" exited with 1.
Looks like tests are failing on this build. You fixed the api.py but didn't update the script which runs the check (which looks for a blank and sets it). Frankly, could probably remove that default from the shell script but I like keeping it there so people know they can easily override it. Default in the api.py then is probably unnecessary.