[SPARK-1946] Submit stage after (configured number) executors have been registered

Because creating TaskSetManager 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 DAGScheduler submit stage after a few of executors have been registered. 

\# submit stage only after successfully registered executors arrived the number, default value 0
spark.executor.minRegisteredNum = 20

\# whatever registered number is arrived, submit stage after the maxRegisteredWaitingTime(millisecond), default value 10000
spark.executor.maxRegisteredWaitingTime = 5000
@tgravescs 
I think default value the config property should be 0 for keeping consistent with previous version, whatever we use number or percentage. 
In my opinion, using executor number as config property should keep consistent with the args(and conf, env) --num-executors, user  can accept the conf property more easily.