[SPARK-10394] [ML] Make GBTParams use shared stepSize

```GBTParams``` has ```stepSize``` as learning rate currently.
ML has shared param class ```HasStepSize```, ```GBTParams``` can extend from it rather than duplicated implementation.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #8552 from yanboliang/spark-10394.