[SPARK-21383][YARN] Fix the YarnAllocator allocates more Resource

When NodeManagers launching Executors,
the `missing` value will exceed the
real value when the launch is slow, this can lead to YARN allocates more resource.

We add the `numExecutorsRunning` when calculate the `missing` to avoid this.

Test by experiment.

Author: DjvuLee <lihu@bytedance.com>

Closes #18651 from djvulee/YarnAllocate.