User account creation fails on Debian 7.1

```
TASK: [Create main user account] **********************************************
failed: [vps1.lukecyca.com] => {"failed": true, "item": ""}
msg: Group luke does not exist

FATAL: all hosts have already failed -- aborting
```

Simply removing `${main_user_name}` from the list of groups seems to work. Debian creates a primary group with the same name as the user automatically.