[SPARK-8646] PySpark does not run on YARN if master not provided in command line

andrewor14 davies vanzin can you take a look at this? thanks

Author: Lianhui Wang <lianhuiwang09@gmail.com>

Closes #7438 from lianhuiwang/SPARK-8646 and squashes the following commits:

cb3f12d [Lianhui Wang] add whitespace
6d874a6 [Lianhui Wang] support pyspark for yarn-client