[ONOS-6663] Shutdown executor in RestDeviceProvider

Change-Id: If45c0a513104a7cc4eae242f83e3785f30f2a7db
diff --git a/providers/rest/device/src/main/java/org/onosproject/provider/rest/device/impl/RestDeviceProvider.java b/providers/rest/device/src/main/java/org/onosproject/provider/rest/device/impl/RestDeviceProvider.java
index 4963757..a4d9f56 100644
--- a/providers/rest/device/src/main/java/org/onosproject/provider/rest/device/impl/RestDeviceProvider.java
+++ b/providers/rest/device/src/main/java/org/onosproject/provider/rest/device/impl/RestDeviceProvider.java
@@ -179,6 +179,7 @@
         providerService = null;
         factories.forEach(cfgService::unregisterConfigFactory);
         scheduledTask.cancel(true);
+        executor.shutdown();
         log.info("Stopped");
     }