shared.cfg: Move guest_s4 OS dependend setup to guest-os.cfg.sample

These settings were by default in subtests.cfg and windows version
overwrote this in guest-os.cfg. I'd like to move them to guest-os.cfg
for booth versions thus the'll be on the same place and it's gonna
be possible to get them in other tests too.

I put a comment to subtests.cfg about required variables.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>