ca-add: validate Subject DN name attributes
    
    If the Subject DN is syntactically valid but contains unrecognised
    name attributes, FreeIPA accepts it but Dogtag rejects it, returning
    status 400 and causing the framework to raise RemoteRetrieveError.
    
    Update the ca-add command to perform some additional validation on
    the user-supplied Subject DN, making sure that we recognise all the
    attributes.
    
    Fixes: https://pagure.io/freeipa/issue/6987
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
    Reviewed-By: Felipe Volpone <felipevolpone@gmail.com>