Option to proxy a specific cluster, and that cluster only

[alexbrasetvik]

To deploy Sense on Found, we need to be able to have the proxy only proxy requests to a pre-configured cluster, i.e. the cluster that Kibana is pointing at. Currently it can connect to anything, which makes it a risk to deploy.

In that mode, it would make sense to hide the server form, and just have it connect to the cluster it is attached to.

[bleskes]

Note that we added some configuration options to lock down the proxy in #66 . I'm still leaving this one open as we can do better on the UI side.

[jtharpla]

Agreed, it would be useful in hosting scenarios to have a configuration option to disable / hide the server form and lock Sense to a preconfigured cluster, ideally the same cluster as Kibana is attached to.