`tests/conf.py` is a mess

[tobixen]

• The file does no longer reflect much "confguration purpose" and should probably be renamed or split.
• The file has become very big now due to the docker test servers added. Can any of this code be consolidated, any common code there? Maybe move out docker-related code to some python file in the docker directory?
• It should not be needed with the client-method there, davclient.get_davclient should be used instead.

[tobixen]

Quite some work has been done in #578

There are some comments at the top of the file with suggestions for further process.