If the python script run by pvtkpython called sys.exit() explicitly, the
MPI controller cleanup logic wouldn't kick in and the controller would
leak causing test failures. Adding a Py_AtExit() handler ensures that
the controller is cleaned up when the Python interpreter is finalized,
one way or the other.

Change-Id: I5433fcc111b20b3f7f9809d4d8f61c1128b216e2