Define matrix_grafana_dashboard_download_urls_all variable in matrix-grafana role

We shouldn't be using it in the role (`tasks/setup.yml`) without
defining at least some default value in the role itself.

We've always had the override in `group_vars/matrix_servers`,
so the variable was essentially defined (at the playbook level), but
that's not the right way to do things.

roles/matrix-grafana/defaults/main.yml

@@ -16,6 +16,10 @@ matrix_grafana_dashboard_download_urls:
   - "https://raw.githubusercontent.com/matrix-org/synapse/master/contrib/grafana/synapse.json"
   - "https://raw.githubusercontent.com/rfrail3/grafana-dashboards/master/prometheus/node-exporter-full.json"
 
+# matrix_grafana_dashboard_download_urls_all contains the full list (of URLs) of dashboards to download.
+# This variable is overriden elsewhere and additional URLs are injected into it (besides those seen in `matrix_grafana_dashboard_download_urls`) based on the enabled components.
+matrix_grafana_dashboard_download_urls_all: "{{ matrix_grafana_dashboard_download_urls }}"
+
 matrix_grafana_base_path: "{{ matrix_base_data_path }}/grafana"
 matrix_grafana_config_path: "{{ matrix_grafana_base_path }}/config"
 matrix_grafana_data_path: "{{ matrix_grafana_base_path }}/data"