hetzner: Output NixOS config when installing.

This has changed in nixpart version 0.3.0, so it won't output anything
by default, only when requested with -p.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
nix/hetzner: Don't include grub devices.

This has changed in nixpart version 0.3.0 as well, and the new version
is able to output the detected GRUB devices by itself.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>