Add user for unattended JeOS installations

In the same way a user is added when performing a "desktop" unattended installation, let's ensure a user is added when user-login and user-password is set.

I've taken the path to do explicit check for both user-login and user-password as we don't want to create passwordless users for JeOS.

All the needed attributes are marked as "optional" as we do not want to force admins to create a new user.