Setup users with systemd-sysusers

At least for nocloud I want to enable the first user experience (#57 (closed)). This only works of there are no users yet (aka /etc/passwd and /etc/shadow are missing) and are created fresh by systemd-sysusers.

This works mostly, from the installed packages only apt, chrony and uuid-runtime (which we can just remove) don't setup users this way.