Hardcoded xrdf path

[dkar-sto]

cumotion_planner.py has the package (isaac_ros_cumotion_robot_description) and path for xrdf files hardcoded.
This makes using the packages as binary impossible if you need to modify or add a configuration.

I think it would be better to let the user configure the full path of the configuration file. For urdf files you have to actually provide a path, so this would also be more consistent.