[SPARK-6327] [PySpark] fix launch spark-submit from python

SparkSubmit should be launched without setting PYSPARK_SUBMIT_ARGS

cc JoshRosen , this mode is actually used by python unit test, so I will not add more test for it.

Author: Davies Liu <davies@databricks.com>

Closes #5019 from davies/fix_submit and squashes the following commits:

2c20b0c [Davies Liu] fix launch spark-submit from python