Allow to override the default ansible-playbook command

Currently ansible-playbook is hardcoded as:

```
let cmd = ["ansible-playbook", playbook]
```

I need it to prefix with `wsl` on Windows.

Would be good to allow some variable to override the default command, e.g.

```
cmd = os.environ.get('ANSIBLE_PLAYBOOK', 'ansible-playbook').split() + [playbook]
```

?