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).
Fix LVM installation warning and location

As grub2 supports booting kernel on a LVM partition, remove the warning
message that is obseleted. Also the install location is refined to
restrict on mbr if the partition boot record is not allowed to install,
like in this case or dm raid as the partition boot record is used as
data or meta data blocks and blocklist addressing won't work either.
fix user setting in grub2 not written

The user setting is overwritten by the proposed setting during the
installation, we have to check the was_proposed flag and will not allow
repropose new value unless it's set to false.
Merge pull request #17 from WenhuaChang/master

Misc bug fixing and cleanup for 13.1 factory.