Cannot add user to group 'rkt'

[crawford]

This is due to the fact that the 'rkt' group doesn't exist when Ignition is run - it shows up later (created by systemd-sysusers). Ignition should probably use sysuser configs instead of using usermod.

[marineam]

iirc using sysusers was the initial idea but that was dropped because sysusers doesn't support setting some fields like shell. Seems reasonable to extend sysusers though :)

[crawford]

Refer to #1920 instead.