Provide a default PYSPARK_PYTHON for python/run_tests

Without this the version of python used in the test is not
recorded. The error is,

   Testing with Python version:
   ./run-tests: line 57: --version: command not found

Author: Matthew Farrellee <matt@redhat.com>

Closes #2300 from mattf/master-fix-python-run-tests and squashes the following commits:

65a09f5 [Matthew Farrellee] Provide a default PYSPARK_PYTHON for python/run_tests