Graylog federation / multi home

## Expected Behavior
With the move to the Elasticsearch REST Interface since Graylog 2.3 it should be possible to configure multiple elasticsearch cluster to lookup data from. Even if they are located in different locations.

Even better would be to talk to the Graylog API to get only the results back from remote.


## Current Behavior
if you want to have some kind of multi homed Graylog Setup you need to have a Graylog Cluster running on every location and forward all (wanted) logs to a central Graylog to work with that data.

## Context
That is similar to https://github.com/Graylog2/graylog2-server/issues/1004 and the main idea to build a federation. But as we might be able to talk to different elasticsearch cluster that might be true for the Graylog API too.

That way messages did not need to be duplicated and transported from the island into the main graylog.