This allows us to eliminate the companion and decrease overhead for
simple servers which do not use workers.

This also updates validation tasks and documentation, pointing to
variables in the matrix-synapse role which don't currently exist yet
(e.g. `matrix_synapse_container_labels_client_synapse_admin_api_enabled`).

These variables will be added soon, as Traefik labels are added to the
`matrix-synapse` role. At that point, the `matrix-synapse-reverse-proxy-companion` role
will be updated to also use them.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2444

Switching from doing "post-start" loop hacks to running the container
in 3 steps: `create` + potentially connect to additional networks + `start`.
This way, the container would be connected to all its networks even at
the very beginning of its life.

* add prometheus-nginxlog-exporter role

* Rename matrix_prometheus_nginxlog_exporter_container_url to matrix_prometheus_nginxlog_exporter_container_hostname

* avoid referencing variables from other roles, handover info using group_vars/matrix_servers

* fix: stop service when uninstalling

fix: typo

move available arch's into a var

fix: text

* fix: prometheus enabled condition

Co-authored-by: ikkemaniac <ikkemaniac@localhost>

- forego removing Docker images - it's not effective anyway, because it
  only removes the last version.. which is a drop in the bucket, usually

- do not reload systemd - it's none of our business. `--tags=start`,
  etc., handle this

- combine all uninstall tasks under a single block, which only runs if
  we detect traces (a leftover systemd .service file) of the component.
  If no such .service is detected, we skip them all. This may lead to
  incorect cleanup in rare cases, but is good enough for the most part.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2090