[SPARK-18361][PYSPARK] Expose RDD localCheckpoint in PySpark

## What changes were proposed in this pull request?

Expose RDD's localCheckpoint() and associated functions in PySpark.

## How was this patch tested?

I added a UnitTest in python/pyspark/tests.py which passes.

I certify that this is my original work, and I license it to the project under the project's open source license.

Gabriel HUANG
Developer at Cardabel (http://cardabel.com/)

Author: Gabriel Huang <gabi.xiaohuang@gmail.com>

Closes #15811 from gabrielhuang/pyspark-localcheckpoint.