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'm not sure about this though. It's meant to avoid skipping valid levels trying something not as desirable (tasks in pendingTasksWithNoPrefs to be specific). Please let me know if you think this should be addressed in another JIRA (e.g. with some delay introduced to pendingTasksWithNoPrefs).