Unit tests for Objective Tracker

- Fixed a bug in ObjectiveTracker turned up by tests, it
  could not handle an emptry list (it was checking for null only)
- Implemented tests for topology, resource and leadership
  events

Change-Id: I1318a0e8655f5b44b30d2ecb527f665f13d54243

@@ -178,7 +178,7 @@ public class ObjectiveTracker implements ObjectiveTrackerService {
                 return;
             }
 
-            if (event.reasons() == null) {
+            if (event.reasons() == null || event.reasons().isEmpty()) {
                 delegate.triggerCompile(new HashSet<IntentId>(), true);
 
             } else {