toolbox: test image from the compose where possible
    
    per https://pagure.io/fedora-qa/issue/766 , this is a hole in our
    current test approach: we are testing whatever the current
    'stable' toolbox image is for the release, not the image that
    is produced as part of the compose. This enhances the test to
    test both, when possible. If the var TOOLBOX_IMAGE is set, we
    will first check that a 'normal' `toolbox create` works - i.e.
    that all the toolbox logic works right and it can actually find
    a default image to download - but then we throw that toolbox
    away, download the image (the value of the var is expected to
    be a URL for the image file), register it with skopeo, and then
    recreate the container using that image. Then we proceed with
    the rest of the test as usual.
    
    If TOOLBOX_IMAGE is not set, the test should proceed as before,
    using the 'default' downloaded image.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>