Use a method to add repos that will apply $arch and $basearch
substitution automatically. Yum backend only applies $basearch, so if
compatibility is needed, that one should be used.
Drop code for handling mirrorlist, since Pungi does not ever use it, and
being used externally is not really supported.
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>