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.

This is an attempt at optimizing service startup.

The effect is most pronounced when many services are restarted one by one.
The systemd service manager role sometimes does this - for example when `just install-service synapse` runs.
In such cases, a 5-second delay for each Synapse worker service
(or other bridge/bot service that waits on the homeserver) quickly adds up to a lot.

When services are all stopped fully and then started, the effect is not so pronounced, because
`matrix-synapse.service` starts first and pulls all worker services (defined as `Wants=` for it).
Later on, when the systemd service manager role "starts" these worker services, they're started already.
Even if they had a 5-second wait each, it would have happened in parallel.