Making days optional in _get_cluster_config

[brand-fabian]

For #17:

Fix _get_cluster_configuration by adding a default value and making the days capture group otherwise optional.

In order to get the unit tests running with newer versions of the slurm image I had to do some modification to the setup_sacctmgr function, to include a shorter queue and also remove the gres gpu lines present in the slurm config.

Note on test-environment.yml:

• The pytest_namespace hook got deprecated in pytest 4.x (iirc), so I had to use version 3.10.1
• python>3.6 is not installed in the slurm image, therefore I pinned that version and added the link_python function, s.t. python can be found by slurm_status.py in the test environment.
• pytest_cookies>0.3 does not export its Cookies class anymore, so I pinned that version as well.

[percyfal]

Hi,

thanks for looking into the issues regarding the unit tests and updated packages. I had started to write a fix prior to the summer holidays but didn't have time to finish them then. Merging these fixes will resolve the issue of pytest_namespace and other things. See additional comments to code.

Cheers