[SPARK-4891][PySpark][MLlib] Add gamma/log normal/exp dist sampling to P...

...ySpark MLlib

This is a follow up to PR3680 https://github.com/apache/spark/pull/3680 .

Author: RJ Nowling <rnowling@gmail.com>

Closes #3955 from rnowling/spark4891 and squashes the following commits:

1236a01 [RJ Nowling] Fix Python style issues
7a01a78 [RJ Nowling] Fix Python style issues
174beab [RJ Nowling] [SPARK-4891][PySpark][MLlib] Add gamma/log normal/exp dist sampling to PySpark MLlib