[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
ultimately I would prefer the end user config for yarn to be a percentage. That makes it easier to set a decent default across the entire cluster (other then just off).    I guess if we had to we could have another config for yarn which ends up being applied to the # of executors requested and sets this conf for the user, but that is just one more config.