Drop puavo-specific stuff from here, just look at ALLOW_GUESTLOGIN
lts.conf -variable.
Handle the check if guest logins are okay in screen.d/lightdm.
This creates a dependency to Puavo.  Lightdm works without
it as well, but one can not change guest login feature from lts.conf.
This is due to issues on lts.conf evaluation and device.json fetch
order.