Changed the way to end the tests   /  make the mtbf.py cleaner by PEP8

1. os._exit(0) can end the test without program. I don't like the exception stack MTBF driver used to have
2. modified mtbf.py according to PEP8. If you want to check that your python program go accord with PEP8 standard. You can try to apt-get the pep8.