Shutting down executor immediately to avoid evaluation of queued up tasks.

Change-Id: I4a8c2287fa4a760b9a840f681f22a7212d850ae8

@@ -87,7 +87,7 @@ public class VirtualNetworkTopologyProvider extends AbstractProvider implements
     @Deactivate
     public void deactivate() {
         topologyService.removeListener(topologyListener);
-        executor.shutdown();
+        executor.shutdownNow();
         executor = null;
         providerRegistry.unregister(this);
         providerService = null;