[SPARK-20764][ML][PYSPARK] Fix visibility discrepancy with numInstances and degreesOfFreedom in LR and GLR - Python version

## What changes were proposed in this pull request?

SPARK-20097 exposed degreesOfFreedom in LinearRegressionSummary and numInstances in GeneralizedLinearRegressionSummary. Python API should be updated to reflect these changes.

## How was this patch tested?
The existing UT

Author: Peng <peng.meng@intel.com>

Closes #18062 from mpjlu/spark-20764.