feat: Use proper subject DNs in plugins.security.nodes_dn

[siegfriedweber]

Description

Replace the generic subject DN in the configuration setting plugins.security.nodes_dn with the FQDNs of the OpenSearch nodes.

Part of stackabletech/secret-operator#617

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Author

• Helm chart can be installed and deployed operator works
• Integration tests passed: https://testing.stackable.tech/job/opensearch-operator-it-custom/38/
• Release note snippet added: feat: Use proper subject DNs in plugins.security.nodes_dn #144 (comment)

Reviewer

• Code contains useful comments
• Code contains useful logging statements
• (Integration-)Test cases added
• Documentation added or updated. Follows the style guide.
• Changelog updated
• Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

• Feature Tracker has been updated
• Proper release label has been added
• Links to generated (nightly) docs added
• Release note snippet added
• Add type/deprecation label & add to the deprecation schedule
• Add type/experimental label & add to the experimental features tracker

[NickLarsenNZ]

LGTM

[siegfriedweber]

Release notes

Platform improvements

OpenSearch

The verification of the OpenSearch nodes' certificates has been extended. The nodes' certificates now also include the FQDNs of the nodes, which are verified by the other nodes using the plugins.security.nodes_dn configuration setting. Previously, it was only verified that the certificates were issued by the Secret Operator.