Allow to correct upgrade repos (bnc#889791)

Old style client. Writing proper tests would mean quite a big refactoring (believe me, I started doing it), which looks like a too big change for maintenance (most likely to be distributed as DUD for SLE12). I agreed with @kobliha that in order to merge it should be manually tested by me (already done) and another person.

So I'm not only looking for code review but also for manual testing.
I don't remember why I decided to use name instead of alias, but probably lines like
these https://github.com/ancorgs/yast-installation/blob/fix_upgrade_urls/src/clients/inst_upgrade_urls.rb#L438
or these https://github.com/ancorgs/yast-installation/blob/fix_upgrade_urls/src/clients/inst_upgrade_urls.rb#L922
made me think that name was more trustworthy. In several parts of the code it looks like the existence of name is assumed while alias is treated like something optional.