Make it possible to have a custom logconfig

[Glutexo]

Getting logging configuration file name from an environment variable would allow using a custom configuration on local development. Like that, editing the checked-in logconfig.ini would be no longer necessary and these modified files would get accidentally committed. That already happened a few times.

This environment variable would be loaded by the run_gunicorn.py script. It’d default to the current logconfig.ini.

It’s possible to use the INVENTORY_LOGGING_CONFIG_FILE variable name already mentioned in the README.