[ONOS-6395] (vNet) Defining mastership service

Defines the interface for mastership service for virtual networks.
This service is needed to achieve HA and load balancing.

Change-Id: I5b68511c114ebd818429f4f6eb622dff95dd8630
diff --git a/incubator/api/src/main/java/org/onosproject/incubator/net/virtual/VirtualNetworkService.java b/incubator/api/src/main/java/org/onosproject/incubator/net/virtual/VirtualNetworkService.java
index 88ef56b..a303df2 100644
--- a/incubator/api/src/main/java/org/onosproject/incubator/net/virtual/VirtualNetworkService.java
+++ b/incubator/api/src/main/java/org/onosproject/incubator/net/virtual/VirtualNetworkService.java
@@ -110,6 +110,7 @@
      * <li>{@link org.onosproject.net.flow.FlowRuleService}</li>
      * <li>{@link org.onosproject.net.flowobjective.FlowObjectiveService}</li>
      * <li>{@link org.onosproject.net.intent.IntentService}</li>
+     * <li>{@link org.onosproject.mastership.MastershipService}</li>
      * </ul>
      *
      * @param networkId    network identifier