extra_iso: Include extra_files.json metadata
    
    This should be on top level of the ISO, and list files added
    specifically to the ISO. If there's anything inherited from one some
    variant, the files will be listed in metadata in the variant directory.
    
    JIRA: COMPOSE-3069
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>