ostree: Add notification handler to wait for signature
    
    This script can be used as a notification handler. For most messages it
    does nothing, but when it sees a new commit in an ostree repo, it will
    wait for a signature of the new commit to appear.
    
    This is useful for building images later so that they include the
    signature as well.
    
    Fixes: https://pagure.io/pungi/issue/650
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>