add support for systemd credentials

[lnussel]

systemd-firstboot supports a number of credentials to provision machines on first boot:

passwd.hashed-password.root
passwd.plaintext-password.root
passwd.shell.root
firstboot.locale
firstboot.locale-messages
firstboot.keymap
firstboot.timezone

Would be good to support that also in jeos-firstboot

[Vogtinator]

119 got merged