[ros] one class per component stream

Same as #243 without breaking others middlewares (backward compatible)
It works only for `testing/middlewares/ros/video_camera.py` at the moment.

If approved, I will rewrite all ROS streams (and associated `morse.builder.data`)

(see https://github.com/pierriko/morse/compare/feature-rosclasses for an up-to-date compare view)