use config spark.scheduler.priority for specifying TaskSet's priority on DAGScheduler

https://issues.apache.org/jira/browse/SPARK-2618
This `if..else` doesn't actually do anything.  This whole comparator is needlessly complex.  If the intent is lexicographical ordering with higher priority ahead of lower priority, lower jobId ahead of higher jobId, and lower stageId ahead of higher stageId, then this should be sufficient:
```scala
private[spark] class FIFOSchedulingAlgorithm extends SchedulingAlgorithm {
  override def comparator(s1: Schedulable, s2: Schedulable): Boolean = {
    import scala.math.Ordering.Implicits._
    (s1.priority, s2.jobId, s2.stageId) > (s2.priority, s1.jobId, s1.stageId)
  }
}
```