Check controller_manager API early. Fast shutdown.

- Check for all services required by spawner at the beginning, so it can know
  early on that it has all its requisites.

- Remove service waiting from shutdown to ensure a fast teardown.
  Usecase: A spawner that dies after the controller manager should not wait
  for services to appear as they will never appear, the controllers are already
  stopped. This happens for example when killing a Gazebo session.
Fix broken unspawner script.