Starts a new stress process with a given cmd

def start_stress(self, stress_cmd):
        """ Starts a new stress process with a given cmd """
        with open(os.devnull, 'w') as dev_null:
            try:
                stress_proc = subprocess.Popen(stress_cmd, stdout=dev_null,
                                               stderr=dev_null)
                self.set_stress_process(psutil.Process(stress_proc.pid))
            except OSError:
                logging.debug("Unable to start stress")