Basic auth

rack-reverse-proxy supports basic auth, so this essentially passes on a config.

The `preserve_host`-option is also important, as the cluster can be running behind something that cares about the host. 
I'm OK with that change too but it will require some refactoring in the Javascript code. I think the best way to handle this would be to make changes to everything except `elasticsearch`. Then file an issue to have a Kibana team member change `elasticsearch` to `elasticsearch_url`.