Allow 'nfs:NONE' in global configuration

This patch adds 'nfs:NONE' as an allowed entry for the global
authorization data type in the CLI and WebUI. This is an ad-hoc solution
to make sure that the new default value for the NFS service is not
removed by chance.

This patch should be removed if a more generic solution is implemented
to modify service:TYPE style values of the authorization data type.

https://fedorahosted.org/freeipa/ticket/2960

ipalib/plugins/config.py

@@ -194,7 +194,7 @@ class config(LDAPObject):
             cli_name='pac_type',
             label=_('Default PAC types'),
             doc=_('Default types of PAC supported for services'),
-            values=(u'MS-PAC', u'PAD'),
+            values=(u'MS-PAC', u'PAD', u'nfs:NONE'),
             csv=True,
         ),
     )