add a delete vBNG feature

Change-Id: I21ce31d645be31320d06c36795e6b044f11924ab
diff --git a/apps/virtualbng/src/main/java/org/onosproject/virtualbng/VbngService.java b/apps/virtualbng/src/main/java/org/onosproject/virtualbng/VbngService.java
index 4ac7606..c27f86f 100644
--- a/apps/virtualbng/src/main/java/org/onosproject/virtualbng/VbngService.java
+++ b/apps/virtualbng/src/main/java/org/onosproject/virtualbng/VbngService.java
@@ -36,4 +36,13 @@
      */
     IpAddress createVbng(IpAddress privateIpAddress);
 
+    /**
+     * Deletes a virtual BNG.
+     *
+     * @param privateIpAddress a private IP address
+     * @return the public address assigned for this private IP address if the
+     *         virtual BGN exists and is deleted, otherwise return null if
+     *         there is no vBNG for this private IP address
+     */
+    IpAddress deleteVbng(IpAddress privateIpAddress);
 }