automatically disable `auto_create_index` for Graylog managed indices in Elasticsearch

Fresh installations run regular in the problem that the `deflector` alias is created as an index.

We have documented what you need to configure to avoid that plus how you can resolve the situation.

http://docs.graylog.org/en/3.1/pages/faq.html#how-do-i-fix-the-deflector-exists-as-an-index-and-is-not-an-alias-error-message

In the mean elastic has improved the ability to set some settings via API and not only via configuration.

That is why I purpose that Graylog will set the index creation for the indices Graylog manage to off automatically:

https://www.elastic.co/guide/en/elasticsearch/reference/6.8/docs-index_.html#index-creation

This way we would solve the problem users run into - either because they already had elasticsearch running and just blindly point Graylog to it or because they do not notice that in the documentation.