[DPE-3954] Final cleanup

[juditnovak]

Response to the open questions in the related ticket DPE-3954:

COS integration:

• Kibana has an (outdated (*)) Prometheus integrator module, which --exactly as for Opensearch-- requires specific user that can connect to the Dashboard. From what I undrestand from the context, this is rather to be a user of the Dasboard, and not an Opensearch user.

  • Overall it seems to me as a potential source of duplicating work in the near future, if we remove local users from the Dashboard, we should rather keep it
  • (*) NOTE: If the outdated exporter module is replaced with a new one, it's more than likely to use a very similar way of access

• adopting Openserach YAML setter (see [DPE-2942] Charm base #1 (comment)))

  • Still no particular need for that, can be done if a functionality requires this specific implementation. Until that moment there is no harm in holding on to the current 'keep-it-simple' flat implementation