[ONOS-5170] GroupStore: add purgeGroupEntries

The GroupStore exposes purgeGroupEntry, which purges
from the store by a specific device.

Add purgeGroupEntries, to purge entries from all devices
from the GroupStore, and expose purgeGroupEntries to allow
applications to purge all group entries from the GroupStore
without specifying a device.

Change-Id: I735f011a1fbbfa3ce8f1dd57a591a81c4377b012
diff --git a/core/api/src/main/java/org/onosproject/net/group/GroupStore.java b/core/api/src/main/java/org/onosproject/net/group/GroupStore.java
index d2dd992..2cf4aa5 100644
--- a/core/api/src/main/java/org/onosproject/net/group/GroupStore.java
+++ b/core/api/src/main/java/org/onosproject/net/group/GroupStore.java
@@ -125,6 +125,11 @@
     void purgeGroupEntry(DeviceId deviceId);
 
     /**
+     * Removes all group entries from store.
+     */
+    default void purgeGroupEntries() {};
+
+    /**
      * A group entry that is present in switch but not in the store.
      *
      * @param group group entry