Add push VLAN treatment and use it in BgpRouter groups

Change-Id: I8c241fd776cdddd77969413736bd786c0d5a4828
diff --git a/core/api/src/main/java/org/onosproject/net/flow/instructions/L2ModificationInstruction.java b/core/api/src/main/java/org/onosproject/net/flow/instructions/L2ModificationInstruction.java
index 2a93b45..e29ffb9 100644
--- a/core/api/src/main/java/org/onosproject/net/flow/instructions/L2ModificationInstruction.java
+++ b/core/api/src/main/java/org/onosproject/net/flow/instructions/L2ModificationInstruction.java
@@ -15,14 +15,14 @@
  */
 package org.onosproject.net.flow.instructions;
 
-import static com.google.common.base.MoreObjects.toStringHelper;
-
-import java.util.Objects;
-
 import org.onlab.packet.MacAddress;
 import org.onlab.packet.MplsLabel;
 import org.onlab.packet.VlanId;
 
+import java.util.Objects;
+
+import static com.google.common.base.MoreObjects.toStringHelper;
+
 /**
  * Abstraction of a single traffic treatment step.
  */
@@ -80,7 +80,12 @@
         /**
          * VLAN Pop modification.
          */
-        VLAN_POP
+        VLAN_POP,
+
+        /**
+         * VLAN Push modification.
+         */
+        VLAN_PUSH
     }
 
     // TODO: Create factory class 'Instructions' that will have various factory