[Example 15] Using multiple controller managers under different namespaces (backport #423)

[mergify[bot]]

A rewrite of #170 including https://github.com/StoglRobotics-forks/ros2_control_demos/pull/8

An open questions for me:

• It uses the rrbot_base.launch.py: Should we move it to the description package? I deleted it once for the other examples for sake of simplicity. But here it makes sense IMHO.

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  This is an automatic backport of pull request #423 done by Mergify.