Allow to override celery config file name via the SWH_CONFIG_FILENAME env var
this will take precedence over the implicit config file scheme. The expected config file given via the environment variable is expected to have a [celery] section which will be used as config for the Celery app created in swh.scheduler.celery_backend.config.
Related to swh-core#1410 and swh-core#826 (closed).
Migrated from D1067 (view on Phabricator)