ONOS-958: Add a Group action to TrafficTreatment subsystem.

Change-Id: I2a377508b9721ee96a5a52b0bb2ed3a960c982ce
diff --git a/core/api/src/main/java/org/onosproject/net/flow/DefaultTrafficTreatment.java b/core/api/src/main/java/org/onosproject/net/flow/DefaultTrafficTreatment.java
index 90f8c4e..6b5af75 100644
--- a/core/api/src/main/java/org/onosproject/net/flow/DefaultTrafficTreatment.java
+++ b/core/api/src/main/java/org/onosproject/net/flow/DefaultTrafficTreatment.java
@@ -20,6 +20,7 @@
 import org.onlab.packet.IpAddress;
 import org.onlab.packet.MacAddress;
 import org.onlab.packet.VlanId;
+import org.onosproject.core.GroupId;
 import org.onosproject.net.PortNumber;
 import org.onosproject.net.flow.instructions.Instruction;
 import org.onosproject.net.flow.instructions.Instructions;
@@ -241,6 +242,11 @@
         }
 
         @Override
+        public Builder group(GroupId groupId) {
+            return add(Instructions.createGroup(groupId));
+        }
+
+        @Override
         public TrafficTreatment build() {
 
             //If we are dropping should we just return an empty list?