Fixed problem adding hostgroup into netgroup.

The memberof_netgroup association facet for hostgroup has been
explicitly defined to use the serial associator so it will invoke
the right methods.

Ticket #1737

install/ui/hostgroup.js

@@ -37,6 +37,10 @@ IPA.entity_factories.hostgroup = function() {
             name: 'memberof_hostgroup',
             associator: IPA.serial_associator
         }).
+        association_facet({
+            name: 'memberof_netgroup',
+            associator: IPA.serial_associator
+        }).
         association_facet({
             name: 'memberof_hbacrule',
             associator: IPA.serial_associator,