Revert update and workaround package download and ISO creation
    
    This 'scheduler injects ISOs of packages into the tests' approach
    was intended to speed things up, especially for large updates,
    and it did, but it had a few drawbacks. It means restarting
    older tests from the web UI doesn't work as the ISOs get garbage
    collected (you have to re-schedule in this case). And it has the
    rather large problem that you can now only schedule tests from
    the openQA server (or at least a machine with the openQA asset
    share mounted), because the package download and ISO creation
    just happen wherever the scheduler is running and assume that
    the openQA asset share that will be used by the tests is at
    /var/lib/openqa/share in that filesystem.
    
    That's too big of a drawback to continue with this approach, IMO,
    so this reverts back to the old way of doing things.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>