[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/store/dist/src/test/java/org/onosproject/store/group/impl/DistributedGroupStoreTest.java b/core/store/dist/src/test/java/org/onosproject/store/group/impl/DistributedGroupStoreTest.java
index 288d8de..a3ca1ec 100644
--- a/core/store/dist/src/test/java/org/onosproject/store/group/impl/DistributedGroupStoreTest.java
+++ b/core/store/dist/src/test/java/org/onosproject/store/group/impl/DistributedGroupStoreTest.java
@@ -236,6 +236,10 @@
         groupStore.purgeGroupEntry(deviceId2);
         assertThat(groupStore.getGroupCount(deviceId1), is(1));
         assertThat(groupStore.getGroupCount(deviceId2), is(0));
+
+        groupStore.purgeGroupEntries();
+        assertThat(groupStore.getGroupCount(deviceId1), is(0));
+        assertThat(groupStore.getGroupCount(deviceId2), is(0));
     }
 
     /**