Split ContinuousScenarioRunner to constant and constant duration

* When 'type' in scenario config is 'constant', then
  use ConstantScenarioRunner.
* When 'type' in scenario config is 'constant_for_duration',
  then use ConstantForDurationScenarioRunner.
* Sample task configs renamed from 'continuous' to 'constant'.
* Renamed continous.py to constant.py.
* Error to specify 'continuous' for 'type' in scenario config.
* Error to specify 'times' and 'duration' in same config.
* Error to omit 'type' from scenario config.
* Error to omit 'duration' when 'type' is constant_for_duration.
* Improved unit tests to include timeout failure mode cases.

bp benchmark-split-continuous-runner
Change-Id: Idb83f5be538bce5740c7cb6211364caaf2d53b7d