Update error message to reflect external config attribute name

If you try to start the agent and the config is missing `monitoring_token` key, you get the following error:

```bash
pi@raspberrypi ~/virgo $ /usr/bin/rackspace-monitoring-agent -d
Tue Nov 26 19:25:18 2013 DBG: Using state directory /var/run/rackspace-monitoring-agent/states
Tue Nov 26 19:25:18 2013 ERR: 'token' is missing from 'config'
```

The problem is that `token` is an internal name and as such, the error message is confusing. I've updated the error message to reflect an external setting name.