'Runs the setup and starts the scenario
:return: A Deferred that fires when the desired scenario is
established (e.g. that a certain load is being applied).
:raise RequestScenarioAlreadyStarted: if the scenario had been
already started.'

def start(self):

    if self.is_started:
        raise RequestScenarioAlreadyStarted()
    else:
        self.is_started = True
    d = self.request.run_setup()
    d.addCallback(self.run_scenario)
    return d
