[ONOS-5797] GroupService: add setBucketsForGroup

Add method to set all buckets from a group, overwriting the
previous group buckets entirely. Useful for edits that before
required two operations: removing the buckets and then adding
others. It can all be done with one OF message in the end.

Change-Id: Ic5669603ed4fd18b8efaa8d0253ab9d7b1e870f5
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 0c0c06c..4559e64 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
@@ -34,7 +34,12 @@
         /**
          * Modify existing group by removing provided information from it.
          */
-        REMOVE
+        REMOVE,
+        /**
+         * Modify existing group entry by setting the provided information,
+         * overwriting the previous group entry entirely.
+         */
+        SET
     }
 
     /**