Introduce SWH_LOG_TARGET to allow switching between console and journal
The current state of the celery worker/utility logging setup makes us log twice: the default value of log_console can't be overridden by the cli flag in celery workers.
The environment variable can be set to journal when running as a systemd service; this shuts down the console logger, and allows the metadata added to log messages to pass through.
Test Plan
hot-patched on worker01, the worker doesn't double-log
Migrated from D1233 (view on Phabricator)