Fix logging logic in rabbitmq collectd plugin

The logic should be:
 - If the log level is verb AND verbose==True, then log
 - If the log level is verb AND verbose!=True, then don't log

What was actually happening was:
- If the log level is verb AND verbose==True, then log
- If the log level is verb AND verbose!=True, then doesn't
  match the `elif` condition
    - At which point it falls through to the `else` condition,
      which logs. :sadface:

So by nesting two if conditions, we match all things that are
tagged as 'verb' in the outer `elif` condition, but only log
them if verbose==True.