Predefine url to let the environment variable overrider do its job

conf/defaults.ini

@@ -65,6 +65,9 @@ host = 127.0.0.1:3306
 name = grafana
 user = root
 password =
+# Use either URL or the previous fields to configure the database
+# Example: mysql://user:secret@host:port/database
+url =
 
 # For "postgres", use either "disable", "require" or "verify-full"
 # For "mysql", use either "true", "false", or "skip-verify".