Because reprepro will not mirror repositories with 0 packages,
xenial-backports, xenial-updates, xenial-security are missing from
AFS. Until we have them populated, we'll only configure APT to use
the xenial mirror.
Change-Id: Id4d788097ae4e1a5d0f13cb84a46fbb9ac9a839e
Signed-off-by: Paul Belanger <pabelanger@redhat.com>