If a check argument starts with :::settings.something::: look up something in the settings hash.

 Same as :::name::: which looks just in the client config
In the case of a check with args --host :::settings.rabbitmq.host::: it will check the sensu settings for settings[:rabbitmq][:host] and pass that to the check.  

The current behavior, which still works, is to pass `:::name:::` to the check if settings[:client][:name] exists.  The prefix `settings` above is to preserve the existing behavior to look up client properties.