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

Change-Id: I4a8c2287fa4a760b9a840f681f22a7212d850ae8
diff --git a/incubator/net/src/main/java/org/onosproject/incubator/net/virtual/impl/VirtualNetworkTopologyProvider.java b/incubator/net/src/main/java/org/onosproject/incubator/net/virtual/impl/VirtualNetworkTopologyProvider.java
index 85b8a0a..d2e0b26 100644
--- a/incubator/net/src/main/java/org/onosproject/incubator/net/virtual/impl/VirtualNetworkTopologyProvider.java
+++ b/incubator/net/src/main/java/org/onosproject/incubator/net/virtual/impl/VirtualNetworkTopologyProvider.java
@@ -87,7 +87,7 @@
     @Deactivate
     public void deactivate() {
         topologyService.removeListener(topologyListener);
-        executor.shutdown();
+        executor.shutdownNow();
         executor = null;
         providerRegistry.unregister(this);
         providerService = null;