[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/GroupService.java b/core/api/src/main/java/org/onosproject/net/group/GroupService.java
index 369cc37..692bde8 100644
--- a/core/api/src/main/java/org/onosproject/net/group/GroupService.java
+++ b/core/api/src/main/java/org/onosproject/net/group/GroupService.java
@@ -113,6 +113,11 @@
     void purgeGroupEntries(DeviceId deviceId);
 
     /**
+     * Purges all group entries.
+     */
+    default void purgeGroupEntries() {};
+
+    /**
      * Deletes a group associated to an application cookie.
      * GROUP_DELETED or GROUP_DELETE_FAILED notifications would be
      * provided along with cookie depending on the result of the