Allow to set log levels via the `SWH_LOG_LEVEL` environment variable

To make it easier to debug code in the Docker stack, allow to set the
general log level through the `SWH_LOG_LEVEL` environment variable.
This can then easily be set in `docker.override.yml` like so:

    swh-lister:
      environment:
        SWH_LOG_LEVEL: "debug"

Click provides the plumbing to do this easily, see:
https://click.palletsprojects.com/en/8.1.x/options/#values-from-environment-variables