Convert the few sections where tabs were used to spaces

In a few sections tabs were used and when this plugin was used on collectd you would see this error on load:
```
python plugin: Error importing module "rabbitmq".
Unhandled python exception in importing module: IndentationError: unindent does not match any outer indentation level (rabbitmq.py, line 128)
python plugin: Found a configuration for the "rabbitmq" plugin, but the plugin isn't loaded or didn't register a configuration callback.
```

Running ```python -m tabnanny rabbitmq.py``` would show a few errors```
```
rabbitmq.py 129 '\tdetails = queue.get("%s_details" % name, None)\n'
```

Now on start/restart of collectd the plugin works correctly.

```
sudo /etc/init.d/collectd restart
 * Restarting statistics collection and monitoring daemon collectd
Configuring RabbitMQ Plugin
Username = True
Password = True
Realm = True
Host = True
Port = True
Configuring RabbitMQ Plugin
Username = True
Password = True
Realm = True
Host = True
Port = True
   ...done.
```

Thanks for this plugin - it's a huge help!