fix Yast live CD installer ignores /etc/default/grub

Fix Yast live CD installer ignores /etc/default/grub. Now we read
/etc/default/grub to provide default values for non proposed settings.
We also made grub theme a non proposed value as it now been edited
either by theme package post scripts or by suse studio appliances.
(bnc#812266)
fix cannot choose install-to-partition for grub2

fix cannot choose install-to-partition for grub2 (bnc#804149).