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-nginx-proxy/templates/nginx-conf.d/matrix-riot-web.conf.j2

@@ -1,6 +1,6 @@
 server {
 	listen 80;
-	server_name {{ hostname_riot }};
+	server_name {{ matrix_nginx_proxy_proxy_riot_hostname }};
 
 	server_tokens off;
 
@@ -25,7 +25,7 @@ server {
     listen 443 ssl http2;
     listen [::]:443 ssl http2;
 
-    server_name {{ hostname_riot }};
+    server_name {{ matrix_nginx_proxy_proxy_riot_hostname }};
 
 	server_tokens off;
 	root /dev/null;
@@ -33,8 +33,8 @@ server {
 	gzip on;
 	gzip_types text/plain application/json application/javascript text/css image/x-icon font/ttf image/gif;
 
-	ssl_certificate {{ matrix_ssl_config_dir_path }}/live/{{ hostname_riot }}/fullchain.pem;
-	ssl_certificate_key {{ matrix_ssl_config_dir_path }}/live/{{ hostname_riot }}/privkey.pem;
+	ssl_certificate {{ matrix_ssl_config_dir_path }}/live/{{ matrix_nginx_proxy_proxy_riot_hostname }}/fullchain.pem;
+	ssl_certificate_key {{ matrix_ssl_config_dir_path }}/live/{{ matrix_nginx_proxy_proxy_riot_hostname }}/privkey.pem;
 	ssl_protocols {{ matrix_nginx_proxy_ssl_protocols }};
 	ssl_prefer_server_ciphers on;
 	ssl_ciphers "EECDH+AESGCM:EDH+AESGCM:AES256+EECDH:AES256+EDH";

roles/matrix-nginx-proxy/templates/nginx-conf.d/matrix-synapse.conf.j2

@@ -1,6 +1,6 @@
 server {
 	listen 80;
-	server_name {{ hostname_matrix }};
+	server_name {{ matrix_nginx_proxy_proxy_matrix_hostname }};
 
 	server_tokens off;
 
@@ -25,7 +25,7 @@ server {
 	listen 443 ssl http2;
 	listen [::]:443 ssl http2;
 
-	server_name {{ hostname_matrix }};
+	server_name {{ matrix_nginx_proxy_proxy_matrix_hostname }};
 
 	server_tokens off;
 	root /dev/null;
@@ -33,8 +33,8 @@ server {
 	gzip on;
 	gzip_types text/plain application/json;
 
-	ssl_certificate {{ matrix_ssl_config_dir_path }}/live/{{ hostname_matrix }}/fullchain.pem;
-	ssl_certificate_key {{ matrix_ssl_config_dir_path }}/live/{{ hostname_matrix }}/privkey.pem;
+	ssl_certificate {{ matrix_ssl_config_dir_path }}/live/{{ matrix_nginx_proxy_proxy_matrix_hostname }}/fullchain.pem;
+	ssl_certificate_key {{ matrix_ssl_config_dir_path }}/live/{{ matrix_nginx_proxy_proxy_matrix_hostname }}/privkey.pem;
 	ssl_protocols {{ matrix_nginx_proxy_ssl_protocols }};
 	ssl_prefer_server_ciphers on;
 	ssl_ciphers "EECDH+AESGCM:EDH+AESGCM:AES256+EECDH:AES256+EDH";

roles/matrix-nginx-proxy/templates/systemd/matrix-nginx-proxy.service.j2

@@ -1,17 +1,12 @@
 [Unit]
-Description=Matrix nginx proxy server
-After=docker.service
-Requires=docker.service
-Wants=matrix-synapse.service
-{% if matrix_corporal_enabled %}
-Wants=matrix-corporal.service
-{% endif %}
-{% if matrix_riot_web_enabled %}
-Wants=matrix-riot-web.service
-{% endif %}
-{% if matrix_mxisd_enabled %}
-Wants=matrix-mxisd.service
-{% endif %}
+Description=Matrix nginx-proxy server
+{% for service in matrix_nginx_proxy_systemd_required_services_list %}
+Requires={{ service }}
+After={{ service }}
+{% endfor %}
+{% for service in matrix_nginx_proxy_systemd_wanted_services_list %}
+Wants={{ service }}
+{% endfor %}
 
 [Service]
 Type=simple