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/35/
• Links to generated (nightly) docs added
• Release note snippet added

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