[SPARK-2154] Schedule next Driver when one completes (standalone mode)

Author: Aaron Davidson <aaron@databricks.com>

Closes #1405 from aarondav/2154 and squashes the following commits:

24e9ef9 [Aaron Davidson] [SPARK-2154] Schedule next Driver when one completes (standalone mode)

core/src/main/scala/org/apache/spark/deploy/master/Master.scala

@@ -751,6 +751,7 @@ private[spark] class Master(
         driver.state = finalState
         driver.exception = exception
         driver.worker.foreach(w => w.removeDriver(driver))
+        schedule()
       case None =>
         logWarning(s"Asked to remove unknown driver: $driverId")
     }