Fix suspicious behavior

Union of allocated resources and available reousrces doesn't seem
to be the result as available resources

Change-Id: I3c8f0b3293f81b329aa06de16635adc006730598

@@ -287,7 +287,7 @@ public class LinkResourceManager implements LinkResourceService {
 
         Set<ResourceAllocation> allocatedRes = allocations.getResourceAllocation(link);
         Set<ResourceRequest> result = Sets.newHashSet(getAvailableResources(link));
-        result.addAll(allocatedRes);
+        result.removeAll(allocatedRes);
         return result;
     }