SPARK-1937: fix issue with task locality

Don't check executor/host availability when creating a TaskSetManager. Because the executors may haven't been registered when the TaskSetManager is created, in which case all tasks will be considered "has no preferred locations", and thus losing data locality in later scheduling.
I guess technically we might have no hosts in this rack, but right now our TaskScheduler doesn't track that. Maybe we should open another JIRA to track it. I can imagine this happening in really large clusters.