Yesterday, I did not have time to investigate this last issue about ROS and Gazebo installation. So I spent a couple of minutes to do it this evening.

The message made no sense since the missing nodes are absolutely common. So my guess was that something remained from a previous install. I found a duplicate gazebo_ros repo in the catkin_ws folder.

So I had to clean the folders /catkin_ws/build, catkin_ws/devel/share, /catkin_ws/devel/lib/python2.7/dist-packages and /catkin_ws/devel/include and remove the sub-folders gazebo_ros, gazebo_msgs, gazebo_plugins, gazebo_ros_control, and gazebo_ros_pkgs.

And tadam, the rrobot is back again and fully functionnal:

The very big news is that now Gazebo and ROS can be installed directly from the debs. Good job guys!

Unfortunately, the youBot is still shaky, so it looks like that the bug described one month ago finally comes from a misconfiguration somewhere.