Storage unify

It really helps with complexity of two classes - https://codeclimate.com/github/yast/yast-bootloader/compare/storage_unify

It also make reading how we propose place for stage1 much cleaner.

It need more tests, but it needs more real target maps, where we do bad proposal and what we want achieve.
OK, so this?
> The proposal is only based on storage information, disregarding any existing values of the output variables (which are respected at other times, in AutoYaST).