If someone is using Dendrite and enabling Grafana, we'll no longer
download Synapse dashboards.

If someone is not using node-exporter, we'll no longer download node
exporter dashboards.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2133

* if variable to bind an exporter container to a host port is set, have matrix-domain.conf (nginx) support this

* manipulate some variables to account for just port numbers or 0.0.0.0 IPs

* Make sure to use the right variable in the init.yml files

* Update roles/matrix-prometheus-node-exporter/tasks/init.yml

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

* Update roles/matrix-prometheus-postgres-exporter/tasks/init.yml

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

* remove extraneous variables and whitespace

Co-authored-by: Luca Bilke <luca@gmail.com>
Co-authored-by: Slavi Pantaleev <slavi@devture.com>

This commit introduces a new role that downloads and installs the
prometheus community postgres exporter  https://github.com/prometheus-community/postgres_exporter.
A new credential is added to matrix_postgres_additional_databases that
allows the exporter access to the database to gather statistics.
A new dashboard was added to the grafana role, with some refactoring
to enable the dashboard only if the new role is enabled.
I've included some basic instructions for how to enable the role in
the Docs section.

In terms of testing, I've tested enabling the role, and disabling
it to make sure it cleans up the container and systemd role.