Introduce method to write msg and get response through OpenFlow.

Change-Id: Ibc92985a98a3b9a178e474f55c117e08b05a8d44
diff --git a/providers/openflow/group/src/test/java/org/onosproject/provider/of/group/impl/OpenFlowGroupProviderTest.java b/providers/openflow/group/src/test/java/org/onosproject/provider/of/group/impl/OpenFlowGroupProviderTest.java
index ba0634a..b951ebc 100644
--- a/providers/openflow/group/src/test/java/org/onosproject/provider/of/group/impl/OpenFlowGroupProviderTest.java
+++ b/providers/openflow/group/src/test/java/org/onosproject/provider/of/group/impl/OpenFlowGroupProviderTest.java
@@ -62,6 +62,7 @@
 import java.util.Collection;
 import java.util.List;
 import java.util.Set;
+import java.util.concurrent.CompletableFuture;
 
 import static org.junit.Assert.*;
 
@@ -265,6 +266,11 @@
         }
 
         @Override
+        public CompletableFuture<OFMessage> writeResponse(Dpid dpid, OFMessage msg) {
+            return null;
+        }
+
+        @Override
         public void processPacket(Dpid dpid, OFMessage msg) {
             eventListener.handleMessage(dpid, msg);
         }