fedora.conf: atomic: move to unified 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/
    
    Signed-off-by: Dusty Mabe <dusty@dustymabe.com>
    (cherry picked from commit cf4bbe812458b36441dbd092f14b6d93de90cb6d)