reversepassproxy.registry-generic.conf: Set up for flatpak-indexer for staging
    
    Add changes required for flatpak-indexer, conditionalized for staging:
    
     * Reverse which of "index with labels" or "index with annotations" is the
       default (make labels the default, since annotations are only used by
       old versions of Flatpak)
     * Add the deltas/ directory which holds deltas between Flatpak versions.
    
    Signed-off-by: Owen W. Taylor <otaylor@fishsoup.net>