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.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/868

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/868

.. and prevent variable name overlap with `matrix-postgres` for the
other variables as well.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/868

- removed build_database_list.yml

- 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

- adjusted env-postgres-backup.j2 to use the new role specific variables