move to unified atomic ostree repo structure
    
    We move to /compose/atomic/repo for the compose time repo and to
    /atomic/repo for the production/user repo. See [1].
    
    [1] https://lists.fedoraproject.org/archives/list/rel-eng@lists.fedoraproject.org/thread/KLN5L33BIR3ZEHC5RIG4NXGO7LT6HBXJ/