Adduser script

The haproxy file needs insertions in the frontends as well as in the backends section, so in this PR I'm catting it together from 3 parts each time you add a user.
What do you think about replacing this line by:
```bash
core.vm.provision :shell, path: "./scripts/adduser.sh", args: HOSTNAME
```
And then, at the line 20: `HOSTNAME=coreos.dev`, and at line 33 `core.vm.hostname = HOSTNAME`

A bit more DRY :) What do you think?