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.
It doesn't make sense to put this here because it will apply to every TaskSet, no matter how late into the application it was submitted, so you'll get a 3-second latency on every TaskSet that is missing one of its preferred nodes. Can we not add this as part of this patch, and simply make the change to put tasks in the node- and rack-local lists even if no nodes are available in those right now? Then later we can update the code that calls resourceOffer to treat tasks that have preferred locations but are missing executors for them specially.