cloud-atomic: Forcibly power off after %post to work around bug
The system loops with:
mkdir: cannot create directory '/oldsys': Read-only filesystem
mkdir: cannot create directory '/oldsys/sys': Read-only filesystem
mount: mount point /oldsys/sys does not exist
...
drauct Warning: Killing all remaining processes
...
--- begin loop ---
Unable to unlink device node for atomicos-root
Unable to unlink device node for live-base
rm: cannot remove /lib/drauct/hooks/shutdown/30-dm-shutdown.sh: Read-only filesystem
rm: cannot remove /lib/drauct/hooks/shutdown/30-md-shutdown.sh: Read-only filesystem
--- end loop ---
Not entirely sure what's at fault here, but this is a quick hack to
get an image building.