server.username is incorrect

The informational output on my build looks like:

```
Follow along by running:
  ssh -l root ec2-XX-XX-XX-XX.compute-1.amazonaws.com 'tail -f /var/log/user-data.log'
```

Which seems to come from use of `server.username` provided by Fog. It may not be a problem with this library, but I wonder why this is `root` and not `ubuntu` which is the login user for this instance.
