[dynamixel_workbench_controllers] Enable to torque off on shutdown

[pazeshun]

We sometimes want to disable torque of dynamixel motors when dynamixel_workbench_controllers is killed in order not to leave the motors torque on and overheat them. With this PR, we can select that behavior by passing torque_off_on_shutdown:=true to dynamixel_controllers.launch. This PR does not change the default behavior.