discarded exceeded completedDrivers

When completedDrivers number exceeds the threshold, the first Max(spark.deploy.retainedDrivers, 1) will be discarded.

Author: lianhuiwang <lianhuiwang09@gmail.com>

Closes #1114 from lianhuiwang/retained-drivers and squashes the following commits:

8789418 [lianhuiwang] discarded exceeded completedDrivers