keep .ostree directory; release_collection.py uses --keep; refactoring
    
    Do not remove the .ostree directory during collection conversion or
    packaging.
    
    Do not use --force with release_collection.py - the ostree preparation
    may write some preliminary data to the collection destination directory
    that we want to keep - instead use the new --keep option.
    
    Refactor paths to use collection_dest_path and collection_build_path