swh-storage: Unify production and development start up
This now:
- load configuration form the SWH_CONFIG_FILENAME
- uses gunicorn (same as in production)
Depends on swh-storage!902 (closed) Related T1533
Test Plan
doco up swh-storage -d doco logs swh-storage everything should be fine
Migrated from D1168 (view on Phabricator)