nixos/tests/grafana: demonstrate how to use the file provider

Yes, it's still a store path, but it's only for demonstration purposes.
Also we now have fewer warnings in the provision test.

nixos/tests/grafana/provision/default.nix

@@ -17,7 +17,7 @@ let
 
         security = {
           admin_user = "testadmin";
-          admin_password = "snakeoilpwd";
+          admin_password = "$__file{${pkgs.writeText "pwd" "snakeoilpwd"}}";
         };
       };
     };