Centralize the postgresql databases used for admin tools into a single dedicated host
(on which we would be able to do more proper backups and monitoring).
This is also the occasion to use the latest postgresql versions (each have their own and
inconsistent versions).
Services:
- hedgedoc (host: bardo, db: postgres-12)
- opnsense (host: inventory, db: postgres-11)
- grafana (host: pergamon, db: postgres-11)
- sentry (host: pergamon, db: postgres-12)
- keycloak (host: kelvingrove, db: postgres-12)
Related to D6871#178665