[SPARK-2714] DAGScheduler logs jobid when runJob finishes

DAGScheduler logs jobid when runJob finishes
Can you do string interpolation instead to make this more readable:
```
val timeElapsed = (System.nanoTime - start) / 1e9
logInfo(s"Job ${waiter.jobId} finished: ${callSite.shortForm}, took $timeElapsed s")
```