STORM-361. Add JSON-P support to Storm UI API.

What if instead of deserializing and re-serializing the cluster conf, we rewrite `json-response` as a keyword function:

```Clojure
(defnk json-response
  [data callback :serialize-fn to-json :status 200]
  {:status status
   :headers (if (not-nil? callback)
                {"Content-Type" "application/javascript"}
                {"Content-Type" "application/json"})
   :body (if (not-nil? callback)
             (add-padding-to-json callback (serialize-fn data))
             (serialize-fn data))})
```

And then for the route to `/api/v1/cluster/configuration`, we can just call:

```Clojure
(GET "/api/v1/cluster/configuration" [& m]
  (json-response (cluster-configuration)
                 (:callback m)
                 :serialize-fn identity))
```