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.
It is still a bad name because it describes its destination, not its origin or meaning. I think it is clearer to remove the method and inline its body `devices_for_redundant_boot.join(",")` instead