Improve the content of default-config.cfg

[gschwind]

Overview

Try to improve the documentation and the content of default-config.cfg to help users.

Related Issue / Discussion

Additional Information

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

[coveralls]

coverage: 84.061%. remained the same
when pulling 7f33918 on gschwind:pr-improve-default-config
into 31bf412 on geopython:main.