Use similar return types for LeadershipService.getCandidates() and LeadershipService.getCandidates(topic)

Change-Id: I9aaea9dfa14e4e9916103a61c8e59290ad656aa7
diff --git a/core/store/dist/src/main/java/org/onosproject/store/consistent/impl/DistributedLeadershipManager.java b/core/store/dist/src/main/java/org/onosproject/store/consistent/impl/DistributedLeadershipManager.java
index 6f498ff..58fd3eb 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/consistent/impl/DistributedLeadershipManager.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/consistent/impl/DistributedLeadershipManager.java
@@ -166,8 +166,8 @@
     }
 
     @Override
-    public Map<String, Leadership> getCandidates() {
-        return ImmutableMap.copyOf(candidateBoard);
+    public Map<String, List<NodeId>> getCandidates() {
+        return Maps.toMap(candidateBoard.keySet(), this::getCandidates);
     }
 
     @Override