Add support for breaking queue collection by vhost.

The current RabbitMQ collector doesn't work well if running multiple virtual hosts with conflicting queue names.  This update shouldn't break people with a current deployment, but will allow a user to add config to split their metrics by vhost.

Basically, adding the following to the RabbitMQ collector configuration should cause collection of all queues split by vhost.

```
[vhosts]
* = *
```

More complex configurations should be possible as well.

```
[vhosts]
* = *
foo = baz bar
vhost2 = queue1 queue2 queue3
```

I've tested a fair amount on my systems attempting to ensure that the update would not break current deployments, so hopefully it should be safe.