Add configs for stg.
This treats the fedora infrastructure staging environment just like
another secondary architecture (at dgilmore's suggestion in
the fedora-releng channel today).
Fixed after initial review on the buildsys list[0].
https://lists.fedoraproject.org/pipermail/buildsys/2015-April/004629.html
Signed-off-by: Ralph Bean <rbean@redhat.com>