The plymouthcfg Calamares module is optional. Distributions which
write filesystems with a full plymouth configuration won't even
want to use it (see plymouthcfg docs).
However, now grubcfg depends on plymouthcfg to run because
the globalstorage value to trigger setting 'splash' in grub,
is set in the plymouthcfg module.
Just check for plymouth existence separately in the grub module.
Fixes ea1c8a0e5d
