Until now, we were leaving services "enabled"
(symlinks in /etc/systemd/system/multi-user.target.wants/).
We clean these up now. Broken symlinks may still exist in older
installations that enabled/disabled services. We're not taking care
to fix these up. It's just a cosmetic defect anyway.
- add matrix_postgres_backup_databases to be build on top of matrix_postgres_additional_databases
- POSTGRES_DB is now directly set from matrix_postgres_backup_databases while building the templates/env-postgres-backup.j2