ros-distro specific directories

[jrgnicho]

There are currently directories for each ros-distribution. This setup requires creating a ros/ros-distro directory with at least a setup.bash and ros_user_library.bash scripts in order to successfully open the terminals. These scripts are practically the same for each ros distribution supported in this repo and so creating a specific set of scripts is unpractical. One solution would be to have ros_session.bash run things on these directories optionally instead of aborting whenever they are not found.