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.
Oh. Unless this will be refactored away next month, can we put this information as a doc comment of `attr :globals` please?