[ROS1/ROS2] Template refactor proposal

[mikaelarguedas]

With the recent migration of EOL distro packages to the snapshot repo, the current set of templates (regular vs legacy) shows its limitations.

To name a couple:

• legacy templates install only the ros package that may not be relevant for distros post-fuerte that would benefit from multiple images (ros-core, ros-base, robot, desktop etc)
• Some ROS distro are still active but with some targeted platform that are EOL (e.g. kinetic/debian jessie). In this case the images based on EOL distros would need to use the snapshots repository.

It appears that now all distros (including active ones) get pushed to the snapshots repositories each sync.
This seems to be a good opportunity for leveraging this to homogenize templates as well as ensuring up-to-date images.
For example:
All images (active or EOL) could use the snapshots repository, the EOL ones would use the final tag while the active ones would use the "sync date" one.
This would allow to provide very consistent builds as well as releasing new docker images at each sync (#112).

What would be needed to make it work:

• buildfarm: upload of all distros to snapshots.ros.org at each sync, and a final one being pushed as soon as the last sync occurs.
• a way for this repo to detect / be triggered on each distro sync
  • then use this trigger to regenerate the images to use the latest sync date (or final)
• automatic PR generation for submitting new images to official library (ala Release hash script and update hashes #230)