grafana: Make the database for dashboards.al.org stateless

We don't have any state for dashboards.archlinux.org we want to persist,
so to avoid any stale data in the database, start with a new empty
in-memory database every time Grafana starts.

roles/grafana/templates/grafana.ini.j2

@@ -106,7 +106,11 @@ enable_gzip = true
 ;server_cert_name =
 
 # For "sqlite3" only, path relative to data_path setting
+{% if grafana_anonymous_access %}
+path = ":memory"
+{% else %}
 ;path = grafana.db
+{% endif %}
 
 # Max idle conn setting default is 2
 ;max_idle_conn = 2