Avoid unconditional wait for Timbuktu screen in _boot_to_anaconda
    
    The way this currently works, the test unconditionally waits 60
    seconds for the "Timbuktu screen" (the warning dialog shown on
    pre-release images) to appear when anaconda is starting up, even
    if it's testing an image where it doesn't show up. Now we test
    Atomic nightlies and live respins and stuff this happens quite a
    lot, so let's avoid it. This way if the hub appears during those
    60 seconds we'll spot it right away and continue, otherwise we
    behave the same as before.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>