consolidate repo option names
    
    live_images:            additional_repos        -> repo
    ostree:                 source_repo_from        -> repo_from
                            extra_source_repos      -> repo
    ostree_installer:       source_repo_from        -> repo_from
    
    With the change, the phases have consolidate option names for variant
    repos and external repos.
    
    Old option names will continue to work, old names will be converted
    to new names after validation automatically if new options are not
    specified in config.
    
    Signed-off-by: Qixiang Wan <qwan@redhat.com>