otptoken-add-yubikey: When --digits not provided use default value
    
    Since Thin client was introduced default values for options are not populated
    in client side plugins. When option has default value and is needed in client
    plugin it must be handled by explicitly.
    
    https://pagure.io/freeipa/issue/6900
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>