Diagnostic-aggregator and diagnostic-updater porting to ROS2

@dirk-thomas : we have created new ros2_devel branch and merged changes from bponsler:ros2-devel branch. Additionally we have updated code and port test cases as per ROS2 guidelines to support ROS2 Bouncy.
Now we want to share this development but there is no branch for ros2 development on https://github.com/ros/diagnostics , so please suggest how to move forward in this case .