Create `odcs-$COMPOSE_ID` symlink pointing to `compose.toplevel_dir`.
    
    In order to stop generating the `latest-*` and
    `odcs-$date-$compose_id.n.0` directories and to switch to `odcs-$compose_id`
    layout, we need to ensure that there is `odcs-$compose_id` directory available
    for already generated composes.
    
    After some transition period and few ODCS releases, we can stop generating
    those old directories and move to `odcs-$compose_id` layout. The old
    composes generated before this upgrade would remain working, because the
    `odcs-$compose_id` symlink will exist.
    
    Signed-off-by: Jan Kaluza <jkaluza@redhat.com>