Unify buildroot repo handling and base it on a variable
    
    This is intended to address the problem that we don't use the
    buildroot repo for Branched between branching and u-t activation,
    which often causes problems when newly-created updates depend on
    builds that recently went stable. So instead of having logic in
    the tests that sets up the buildroot repo only if the release is
    Rawhide (or ELN), we'll do the decision about whether to set it
    up in the scheduler (which decides based on info from Bodhi), and
    just have the 'set up the repo' steps in the tests. Let's also
    use the same code and repo name for ELN and non-ELN so we don't
    have pointless divergence there.
    
    Depends on the matching fedora_openqa change, of course - without
    that, no test will use the buildroot repo.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>