Use release number not "rawhide" for Rawhide upgrades (#1531356)
    
    There's a problem with using `--releasever=rawhide` for upgrade
    tests ATM - see #1531356 . To avoid this, we'll try using the
    real Rawhide release number (which I'm adapting the scheduler
    code to discover and pass in as `RAWREL`).
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>