[SPARK-15957][FOLLOW-UP][ML][PYSPARK] Add Python API for RFormula forceIndexLabel.

## What changes were proposed in this pull request?
Follow-up work of #13675, add Python API for ```RFormula forceIndexLabel```.

## How was this patch tested?
Unit test.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #15430 from yanboliang/spark-15957-python.