This service will be removed soon, but for now we need to get rid of
`matrix_docker_network` usage everywhere.

Same as 250b91a40968e, but for Slack

Postgres is not in `matrix_docker_network` anymore, so what we had
before could not possibly work anymore.

The old variables existed as well, but I inteded to use these new ones.

The user may be running Prometheus elsewhere. It doesn't need to be
getting installed using the current playbook.

Regression since ce2f541deb

This requires at least `v0.14.0-2` of the `prometheus-postgres-exporter`
Ansible role.

This requires at least `v1.7.0-2` of the `prometheus-node-exporter`
Ansible role.