[python alternative] pyspark require Python2, failing if system default is Py3 from shell.py

Python alternative for https://github.com/apache/spark/pull/392; managed from shell.py

Author: AbhishekKr <abhikumar163@gmail.com>

Closes #399 from abhishekkr/pyspark_shell and squashes the following commits:

134bdc9 [AbhishekKr] pyspark require Python2, failing if system default is Py3 from shell.py

(cherry picked from commit bb76eae1b50e4bf18360220110f7d0a4bee672ec)
Signed-off-by: Reynold Xin <rxin@apache.org>