submit stage after (configured ratio of) executors have been registered
support yarn mode(percentage style)
support yarn-cluster
Fix driver lost --num-executors option in yarn-cluster mode
Code style, revert default minRegisteredRatio of yarn to 0, driver get --num-executors in yarn/alpha
Code style, rename configuration property name of minRegisteredRatio & maxRegisteredWaitingTime
Add docs for new configs & code style
Move waitBackendReady from DAGScheduler.submitStage to TaskSchedulerImpl.submitTasks
Move waitBackendReady to TaskSchedulerImpl.start & some code refactor