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.

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