Installation clients

@mvidner how it can be handled without library? I also try to be conform with documented API. Many clients still implement it like: https://github.com/yast/yast-bootloader/blob/master/src/clients/bootloader_proposal.rb#L230
That is why I make it optional, so dispatcher handle it, but can live without it. Basically I implement API: seehttps://github.com/yast/yast-installation/blob/master/doc/proposal_api.md