675 extended env variable interpolation

[nbrinckm]

Overview

This is an example implementation on how the env interpolation could work.

Related Issue / Discussion

See #675

Additional Information

This is currently just the solutiuon as it is in the pycsw. But it still hope it could be helpful for the future.
I'm a little bit worried about the backward compatibility. But as the system doesn't expand values for that there are no env variables it should be okayish for use cases that used the $ syntax for their values in the past.

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[cehbrecht]

@nbrinckm thanks 🙂