nixos/smokeping: Make default imgUrl relative.

This avoids a common problem:

Until now, port forwarding to multiple hosts running smokeping did not work;
they all show the data of the first smokeping instance.
That ws because the image URLs generated by smokeping are absolute
(`imgurl` setting).
Consequently, if you ran
  ssh node-1 -L 8081:localhost:8081
  ssh node-2 -L 8081:localhost:8082
  ssh node-3 -L 8081:localhost:8083
and try to open http://localhost:8081, http://localhost:8082 and
http://localhost:8083, they all would show the images of node-1!

Using a relative `imgurl` fixes that.
As per smokeping docs on `imgurl`:

> Either an absolute URL to the `imgcache` directory or one relative to the
> directory where you keep the SmokePing cgi.

nixos/modules/services/networking/smokeping.nix

@@ -131,10 +131,15 @@ in
       };
       imgUrl = mkOption {
         type = types.str;
-        default = "http://${cfg.hostName}:${toString cfg.port}/cache";
-        defaultText = literalExpression ''"http://''${hostName}:''${toString port}/cache"'';
+        default = "cache";
+        defaultText = literalExpression ''"cache"'';
         example = "https://somewhere.example.com/cache";
-        description = "Base url for images generated in the cgi.";
+        description = ''
+          Base url for images generated in the cgi.
+
+          The default is a relative URL to ensure it works also when e.g. forwarding
+          the GUI port via SSH.
+        '';
       };
       linkStyle = mkOption {
         type = types.enum ["original" "absolute" "relative"];