Switching back from data-node to normal OS should be easy.

## Expected Behavior
When I want to stop using Datanode, in my `graylog.conf` I can modify `elasticsearch_hosts` so that it points to any ES/OS instance, restart GL and have it work with configured ES/OS.

## Current Behavior
After the change, GL does not start, printing a log of `Datanode is not available (...)` messages in the server logs.
In order to force it to stop using Datanode, I have to manually remove some documents in MongoDB collections (like `preflight` collection...).

## Possible Solution
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## Your Environment
* Graylog Version: 6.0
* OpenSearch Version: 2.12
* Operating System: Linux