Copter: easier test of benchtest the parachute

At the moment, because of the land-check in the parachute driver (which helps ensure we don't have unintended deployments on the ground near people) it can be difficult for a user to ensure the parachute servo is working during the setup. We should perhaps clarify the testing procedure on the wiki and/or allow an override of the landing check - perhaps by allowing the mission planner to send a deploy-chute command with a magic number in param2 to specify that the chute should deploy no-matter what!