Configure Grafana's default_home_dashboard_path properly

Using `matrix_synapse_*` variables within the `matrix-grafana` role
is not a good practice.

We now have a `matrix_grafana_default_home_dashboard_path` variable
with a good universal default value and we override it via
`group_vars/matrix_servers` based on enabled components, etc.

This is a better fix for https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2133

group_vars/matrix_servers

@@ -2484,6 +2484,15 @@ matrix_grafana_dashboard_download_urls_all: |
     (matrix_prometheus_postgres_exporter_dashboard_urls if matrix_prometheus_postgres_exporter_enabled else [])
   }}
 
+matrix_grafana_default_home_dashboard_path: |-
+  {{
+    {
+      'synapse': ('/etc/grafana/dashboards/synapse.json' if matrix_synapse_metrics_enabled else '/etc/grafana/dashboards/node-exporter-full.json'),
+      'dendrite': '/etc/grafana/dashboards/node-exporter-full.json',
+      'conduit': '/etc/grafana/dashboards/node-exporter-full.json',
+    }[matrix_homeserver_implementation]
+  }}
+
 matrix_grafana_systemd_wanted_services_list: |
   {{
     []

roles/matrix-grafana/defaults/main.yml

@@ -54,6 +54,10 @@ matrix_grafana_content_security_policy: true
 matrix_grafana_content_security_policy_customized: false
 matrix_grafana_content_security_policy_template: "script-src 'self' 'unsafe-eval' 'unsafe-inline' http: https: 'strict-dynamic' $NONCE;object-src 'none';font-src 'self';style-src 'self' 'unsafe-inline' blob:;img-src * data:;base-uri 'self';connect-src 'self' grafana.com ws://$ROOT_PATH wss://$ROOT_PATH;manifest-src 'self';media-src 'none';form-action 'self';"
 
+# matrix_grafana_default_home_dashboard_path influences the `default_home_dashboard_path` grafana.ini setting,
+# which is an in-container path for the default dashboard.
+matrix_grafana_default_home_dashboard_path: /etc/grafana/dashboards/node-exporter-full.json
+
 # A list of extra arguments to pass to the container
 matrix_grafana_container_extra_arguments: []
 

roles/matrix-grafana/templates/grafana.ini.j2

@@ -26,8 +26,4 @@ enabled = {{ matrix_grafana_anonymous_access }}
 org_name = "{{ matrix_grafana_anonymous_access_org_name }}"
 
 [dashboards]
-{% if matrix_synapse_metrics_enabled %}
-default_home_dashboard_path = /etc/grafana/dashboards/synapse.json
-{% else %}
-default_home_dashboard_path = /etc/grafana/dashboards/node-exporter-full.json
-{% endif %}
+default_home_dashboard_path = {{ matrix_grafana_default_home_dashboard_path }}