Executor initialization moved to activate() method

Change-Id: I91922b467794d25405c2c11cf2fc626efbaa4f22
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 a4d9f56..37b3d02 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
@@ -130,8 +130,7 @@
     private DeviceProviderService providerService;
     private ApplicationId appId;
 
-    private final ExecutorService executor =
-            Executors.newFixedThreadPool(5, groupedThreads("onos/restsbprovider", "device-installer-%d", log));
+    private ExecutorService executor;
     private SharedScheduledExecutorService portStatisticsExecutor = SharedScheduledExecutors.getPoolThreadExecutor();
 
     protected final List<ConfigFactory> factories = ImmutableList.of(
@@ -164,6 +163,7 @@
         appId = coreService.registerApplication(APP_NAME);
         providerService = providerRegistry.register(this);
         factories.forEach(cfgService::registerConfigFactory);
+        executor = Executors.newFixedThreadPool(5, groupedThreads("onos/restsbprovider", "device-installer-%d", log));
         cfgService.addListener(cfgLister);
         executor.execute(RestDeviceProvider.this::createAndConnectDevices);
         executor.execute(RestDeviceProvider.this::createDevices);