StorageService API for creating AsyncDocumentTree primitive

Change-Id: Ib7c3f19beb7b26a5b69161cf972c3c64d0be94b3
diff --git a/apps/vtn/vtnrsc/src/test/java/org/onosproject/vtnrsc/util/VtnStorageServiceAdapter.java b/apps/vtn/vtnrsc/src/test/java/org/onosproject/vtnrsc/util/VtnStorageServiceAdapter.java
index a04ef98..ebad932 100644
--- a/apps/vtn/vtnrsc/src/test/java/org/onosproject/vtnrsc/util/VtnStorageServiceAdapter.java
+++ b/apps/vtn/vtnrsc/src/test/java/org/onosproject/vtnrsc/util/VtnStorageServiceAdapter.java
@@ -15,6 +15,7 @@
  */
 package org.onosproject.vtnrsc.util;
 
+import org.onosproject.store.service.AsyncDocumentTree;
 import org.onosproject.store.service.Topic;
 import org.onosproject.store.service.ConsistentTreeMapBuilder;
 import org.onosproject.store.service.WorkQueue;
@@ -81,4 +82,9 @@
     public <T> Topic<T> getTopic(String name, Serializer serializer) {
         return null;
     }
+
+    @Override
+    public <V> AsyncDocumentTree<V> getDocumentTree(String name, Serializer serializer) {
+        return null;
+    }
 }