Add a sample executable that hogs up all the CPU

[emersonknapp]

• Add a new ament_cmake ROS2 package example_nodes
• Create a process that hogs a configurable amount of CPU by forking busy processes
• Copy the new package into the Dockerfile so it is built
• Reorder the Dockerfile a little so it is more friendly to rebuild when code is changed
• Add instructions in README for a demo that makes everything slow to a crawl (and fix up some existing issues in the sandbox instructions)