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.
as opposed to partial proposal based on known information like e.g. autoyast can do. So even if activate is set before this method, it will be ignored and do not try to make proposal that fits request to explicitelly activate partition. Same with generic_mbr which can prevent boot_mbr option.