Cannot create deflector alias in an empty secured Elasticsearch cluster

When making a clean install of the current Graylog version 3.1.x and a clean install of a secure Elasticsearch 6.8.x cluster, Graylog is not able to create the deflector alias. Index maintenance (recalculating index range / rotating indexes) is not helping.

My current workaround is doing a clean install of both Graylog and Elasticsearch and set `xpack.security.enabled` to `false` until Graylog has set up the deflector alias. Then you can enable `xpack.security.enabled` in your Elasticsearch cluster, reconfigure Graylog to use TLS & Basic auth, restart Graylog and things are up and running.

## Expected Behavior
When doing a clean install of both Graylog and secured Elasticsearch, Graylog should be able to create deflector alias and index data via TLS and basic auth from the beginning.

## Current Behavior
Currently Graylog connects to the Elasticsearch cluster, but is not able to index data. On the _System -> Index Sets_ page, the spinner spins forever. Find the Graylog logs attached to this issue.

## Steps to Reproduce (for bugs)
1. Set up an empty Elasticsearch cluster with `xpack.security.enabled` set to true, Transport and http security enabled and a user created.
2. Set up a empty Graylog instance and configure it to use above cluster. Messages will appear in the logs
3. When disabling `xpack.security.enabled` things are working just fine :-/

## Context
We are doing clean logging cluster installs via Ansible. Every cluster now needs some extra attention and steps to set up.

## Your Environment

* Graylog Version: `3.1.2`
* Elasticsearch Version: `6.8.3`
* MongoDB Version: `4.2.0`
* Operating System: Docker on Debian 9

__Our configuration__
[docker-compose.yaml.txt](https://github.com/Graylog2/graylog2-server/files/3645858/docker-compose.yaml.txt)
[elasticsearch.yaml.txt](https://github.com/Graylog2/graylog2-server/files/3645851/elasticsearch.yaml.txt)
[graylog.log.txt](https://github.com/Graylog2/graylog2-server/files/3645852/graylog.log.txt)