Display list of all available site plugins on /_plugins/ end point

See issue #2664

When you need to know all available site plugins, you can send:


```sh
$ curl localhost:9200/_plugin/
```

And get the following result:

```javascript
{
  "sites" : [ {
    "name" : "anotherplugin",
    "url" : "/_plugin/anotherplugin/"
  }, {
    "name" : "dummy",
    "url" : "/_plugin/dummy/"
  } ]
}
```