[SPARK-1946] Submit tasks after (configured ratio) executors have been registered

Because submitting tasks and registering executors are asynchronous, in most situation, early stages' tasks run without preferred locality.

A simple solution is sleeping few seconds in application, so that executors have enough time to register.

The PR add 2 configuration properties to make TaskScheduler submit tasks after a few of executors have been registered. 

\# Submit tasks only after (registered executors / total executors) arrived the ratio, default value is 0 
spark.scheduler.minRegisteredExecutorsRatio = 0.8

\# Whatever minRegisteredExecutorsRatio is arrived, submit tasks after the maxRegisteredWaitingTime(millisecond), default value is 30000
spark.scheduler.maxRegisteredExecutorsWaitingTime = 5000
Yeah can you just do that (extract the value as a static constant)?  Then this file can be reduced to just checking for the value of the two environment variables and for the spark.executor.instances conf variable and setting totalExecutors accordingly (spark.worker.instances shouldn't be used I don't think -- see #1214)