More repo baseurl munging, for update upgrade tests to F28 now
    
    Now F28 went stable, we're not disabling updates on upgrade any
    more, and this bug got exposed: the location of the updates and
    updates-testing repos actually changed between F27 and F28, so
    the `baseurl` line from fedora-repos in F27 isn't correct for
    F28. When doing an upgrade from < 28 to > 27, we need to correct
    the URL when we're done installing stuff from the old release
    repos but before we start trying to pull stuff from the new
    release repos.
    
    This repo munging crap is really getting fragile, it'd be great
    if we could get that metadata timing issue resolved so we could
    reliably use mirrormanager...
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>