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

https://issues.apache.org/jira/browse/SPARK-2618
Is the style checker ok with `val priority = if (...) {...` instead of `val priority = { if (...) {...`?  If it is, I'd rather do without the extra `{}`.  You can also drop the `: Int` from `val DEFAULT_PRIORITY` and `val priority` -- the types are obvious without the annotations.  Also, I'm not sure that DEFAULT_PRIORITY really gains you anything -- I'd be fine with just `if (...) {...} else 0`.  And make sure you follow the style guide for spacing with parens and braces.