parameters: introduce no_convert keyword argument

When set to true, the argument causes params to not convert unicode values
to the param type.

This will allow thin client to properly handle params which can be
converted from unicode to the param type only on the server, e.g. because
of a normalizer.

https://fedorahosted.org/freeipa/ticket/4739

Reviewed-By: David Kupka <dkupka@redhat.com>

pylint_plugins.py

@@ -152,6 +152,7 @@ ipa_class_members = {
         'alwaysask',
         'sortorder',
         'option_group',
+        'no_convert',
      ],
     'ipalib.parameters.Bool': [
         'truths',