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.
Nit: should be indented with 2 more spaces:
```
if (!pendingTasksForExecutor.isEmpty && getLocalityWait(PROCESS_LOCAL) != 0 &&
    pendingTasksForExecutor.keySet.exists(sched.isExecutorAlive(_))) {
  levels += PROCESS_LOCAL
}