Redo release vars: add UP{1,2}REL and LABEL, don't set DEVELOPMENT
    
    The DEVELOPMENT var is pretty problematic and sometimes a lie.
    I'm rejigging the tests to not use it, and improve their logic.
    But this will mean we always want CURRREL to be set and always
    be the actual current release (we don't want it to be a lie for
    update tests sometimes). So we drop DEVELOPMENT, invent UP1REL
    and UP2REL to be specifically for the purpose of defining the
    upgrade source releases, and always set CURRREL, PREVREL, UP1REL
    and UP2REL for all tests including update tests. We also change
    the logic for Rawhide upgrade test source versions: we had just
    been using CURRREL and PREVREL, which meant that when there is
    a Branched, we're not really testing the right thing, we're
    testing upgrades from Rawhide-2 and Rawhide-3. This is now
    changed so we always test Rawhide-1 and Rawhide-2, even if
    Rawhide-1 is Branched. We also add a LABEL var which is set if
    the compose has a label, this is also to help the tests decide
    whether to expect prerelease stuff.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>