Diagnostic-aggregator and diagnostic-updater porting to ROS2

@dirk-thomas, Thanks for your feedback. @trainman419 had created a ros2-devel branch under ros/diagnostics to support ROS2. we have raised a PR request https://github.com/ros/diagnostics/pull/91 against it but CI build check has failed (as below) that seems some build env issue as it is triggering ros-indigo-catkin. We have tested it locally and build/test is successful in Bouncy. so please suggest in this case.

$ sudo apt-get install python-catkin-pkg python-rosdep ros-indigo-catkin
$ rosdep install --default-yes --from-paths ./ --ignore-src --rosdistro $CI_ROS_DISTRO

ERROR: the following packages/stacks could not have their rosdep keys resolved
to system dependencies:
diagnostic_updater: Cannot locate rosdep definition for [ament_cppcheck]

The command "rosdep install --default-yes --from-paths ./ --ignore-src --rosdistro $CI_ROS_DISTRO" failed and exited with 1 during