More flexible native6 default DRS

[bouweandela]

Before you start, please read CONTRIBUTING.md.

Tasks

• Create an issue to discuss what you are going to do, if you haven't done so already (and add the link at the bottom)
• This pull request has a descriptive title that can be used in a changelog
• Circle/CI tests pass. Status can be seen below your pull request. If the tests are failing, click the link to find out why.
• Codacy code quality checks pass. Status can be seen below your pull request. If there is an error, click the link to find out why. If you suspect Codacy may be wrong, please ask by commenting.
• Please use yamllint to check that your YAML files do not contain mistakes

If you need help with any of the tasks above, please do not hesitate to ask by commenting in the issue or pull request.

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Closes #641

[bouweandela]

I'll add some documentation after ESMValGroup/ESMValTool#1652 is merged.