Do not reorder baseurls by storing them in a std::set (SLE12 as well!)

As we are going to support multiple baseurls in a repo file, and the order of appearance in the .repo file expresses a preference. Re-odering the urls by storing them in a `std::set` is not appropriate. Just follow `zypp::RepoInfo::url_set` (will change from `set` to `list`).

Please cherry-pick this for __SLE12 as well__. No need to do it for older distros.