Elasticsearch - signed request for AWS Elasticsearch

## What?
To allow the authentication in AWS Elasticsearch Graylog would need to enable signed request.

Details on that can be found here: https://aws.amazon.com/blogs/security/how-to-control-access-to-your-amazon-elasticsearch-service-domain/

## Why?
If all infrastructure is AWS only you want to adopt best practice in all places.

## Your Environment

* Graylog Version:3.3