[ONOS-3117] Adding listener of Port-Pair-Group

Change-Id: I6684d1126c7eec32f84729a522dc1168860f0fa6

+/*
+ * Copyright 2015 Open Networking Laboratory
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.onosproject.vtnrsc.portpairgroup;
+
+import org.onosproject.event.EventListener;
+
+/**
+ * Entity capable of Port-Pair-Group related events.
+ */
+public interface PortPairGroupListener extends EventListener<PortPairGroupEvent> {
+
+}