Allow RDN changes for users, groups, rolegroups and taskgroups.

To do a change right now you have to perform a setattr like:

ipa user-mod --setattr uid=newuser olduser

The RDN change is performed before the rest of the mods. If the RDN
change is the only change done then the EmptyModlist that update_entry()
throws is ignored.

ticket 323

ipalib/plugins/group.py

@@ -90,6 +90,7 @@ class group(LDAPObject):
         'member': ['user', 'group'],
         'memberof': ['group', 'netgroup', 'rolegroup', 'taskgroup'],
     }
+    rdnattr = 'cn'
 
     label = _('User Groups')