Default to elasticsearch node ID in watermark notifications

## What?
We are showing hostname or IP address for node identification in watermark notifications, but that leaks potentially sensitive information and in some clusters might actually not be overly helpful (e.g. AWS ES).
Default to using the node ID instead, with potentially adding host/ip information as it is available.

## Why?
Host name/IP might not be stable enough and/or there might be multiple nodes running on a single machine. The node ID already is a unique identifier and can be used in API calls to find the actual machine the node is running on.

See https://github.com/Graylog2/graylog2-server/blob/3cb826f2231906ebaf0c967be90291fa0092151e/graylog2-server/src/main/java/org/graylog2/periodical/IndexerClusterCheckerThread.java#L124

## Your Environment

* Graylog Version: 4.1
* Elasticsearch Version: 7.10
* MongoDB Version: n/a
* Operating System: n/a
* Browser version: n/a