[ros2] Use math.radians() to compute default values in launch-files

[martiniil]

Description

These changes make the choice of the default angular values more understandable.

• Needs release of [Foxy] Evaluate math symbols and functions in python expression (#557) ros2/launch#562

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PR Checklist

Things to add, update or check by the maintainers before this PR can be merged.

• Public api function documentation
• Architecture documentation reflects actual code structure
• Tutorials
• Overview on ROS wiki
• Package Readme (example pilz_robots)
• Good commit messages (some tips)
• CHANGELOG.rst updated
• Copyright headers
• Examples

Review Checklist

• Soft- and hardware architecture (diagrams and description)
• Test review (test plan and individual test cases)
• Documentation describes purpose of file(s) and responsibilities
• Code (coding rules, style guide)

Release planning (please answer)

• When is the new feature released?
• Which dependent packages do have to be released simultaneously?

Hardware tests

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• Perform hardware tests