extra_iso: Stop including variant extra files
    
    They are not always wanted, so let's not include them by default.
    There's a new option to include the same files that extra files phases
    uses, or alternatively they can be configured specifically and put into
    the variant subdirectory.
    
    JIRA: COMPOSE-3084
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>