Do not require description in UI.

Description attribute is not required in LDAP schema so there is no reason to
require it in UI. Modified tests to reflect this change.

https://fedorahosted.org/freeipa/ticket/4387

Reviewed-By: Martin Kosek <mkosek@redhat.com>

ipalib/plugins/privilege.py

@@ -102,7 +102,7 @@ class privilege(LDAPObject):
             label=_('Privilege name'),
             primary_key=True,
         ),
-        Str('description',
+        Str('description?',
             cli_name='desc',
             label=_('Description'),
             doc=_('Privilege description'),