Fix elasticsearch plugin not using auth for cluster stats request.

This fixes a problem where the additional request that the elasticsearch plugin does for cluster stats doesn't use the full `request_kwargs` from the config, causing authentication to fail in scenarios where it is required (found.no hosted Elasticsearch in this case).

This also extends `HTTPStatsPlugin.http_get()` with a `url` parameter for use by any other plugin that needs to grab stats from additional URLs.