composer needs ENV var HOME or COMPOSER_HOME

to run correctly, composer needs one of HOME or COMPOSER_HOME set.
For now, we default to root and set /root as HOME.
in a later iteration when we allow this to run under different users, we
should switch that to

```puppet
     Exec {
       user        => $user,
       environment => "HOME=/home/${user}"
       ...
```

Which will probably be very inadequate...