DNParam: raise Exception when multiple values provided to a 1-val param
    
    When ipa user-add-certmapdata is called with multiple --subject or
    multiple --issuer, the DNParam's _convert_scalar method is called with
    a tuple containing all the params and should raise an exception as the
    --subject and --issuer are single-value params.
    
    The DNParam _convert_scalar method internally calls the DN init method,
    and the DN init method is able to create a DN from a tuple of RDNs.
    As such, it won't raise exception if a tuple/list is provided.
    
    Check that _convert_scalar is only provided a single element.
    
    Fixes: https://pagure.io/freeipa/issue/8097
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>