bundles.cf: add url_ping agent bundle to do HTTP ping

Simple bundle to ping a URL.  @zzamboni @nickanderson please review.

I can change it (removing the reports) so it can be used with `classes` directly, e.g.

```
"ping" usebundle => url_ping(...), classes => if_ok("url_reachable");
```

or I can make it return a value.  WDYT?