Make roles more independent of one another

With this change, the following roles are now only dependent
on the minimal `matrix-base` role:
- `matrix-corporal`
- `matrix-coturn`
- `matrix-mailer`
- `matrix-mxisd`
- `matrix-postgres`
- `matrix-riot-web`
- `matrix-synapse`

The `matrix-nginx-proxy` role still does too much and remains
dependent on the others.

Wiring up the various (now-independent) roles happens
via a glue variables file (`group_vars/matrix-servers`).
It's triggered for all hosts in the `matrix-servers` group.

According to Ansible's rules of priority, we have the following
chain of inclusion/overriding now:
- role defaults (mostly empty or good for independent usage)
- playbook glue variables (`group_vars/matrix-servers`)
- inventory host variables (`inventory/host_vars/matrix.<your-domain>`)

All roles default to enabling their main component
(e.g. `matrix_mxisd_enabled: true`, `matrix_riot_web_enabled: true`).
Reasoning: if a role is included in a playbook (especially separately,
in another playbook), it should "work" by default.

Our playbook disables some of those if they are not generally useful
(e.g. `matrix_corporal_enabled: false`).

roles/matrix-riot-web/templates/systemd/matrix-riot-web.service.j2

@@ -1,7 +1,9 @@
 [Unit]
-Description=Matrix Riot web server
-After=docker.service
-Requires=docker.service
+Description=Matrix riot-web server
+{% for service in matrix_riot_web_systemd_required_services_list %}
+Requires={{ service }}
+After={{ service }}
+{% endfor %}
 
 [Service]
 Type=simple
@@ -12,7 +14,7 @@ ExecStart=/usr/bin/docker run --rm --name matrix-riot-web \
 			-v {{ matrix_riot_web_data_path }}/config.json:/etc/riot-web/config.json:ro \
 			-v {{ matrix_riot_web_data_path }}/home.html:/etc/riot-web/home.html:ro \
 			--network={{ matrix_docker_network }} \
-			{% if not matrix_nginx_proxy_enabled %}
+			{% if matrix_riot_web_container_expose_port %}
 			-p 127.0.0.1:8765:80 \
 			{% endif %}
 			{{ matrix_riot_web_docker_image }}