[opensuse-buildservice] [Patch/RFC] osc: only use needed packages for createrepos

Hi! While doing some kiwi builds with "osc build foo.kiwi", I noticed that it took ages to setup the chroot and also to run createrepo. This is because _all_ repositories in the cache get copied and indexed. This can be more than annoying (read: pita) if you've a big cache. The attached patch is a first shot to use only the needed packages (or repos). Comments ? Best, Jan-Simon