helm: replace existingForSecret property and fix an issue where we could not provide loki configuration with an external ConfigMap

[romainDavaze]

What this PR does / why we need it:

This PR fixes an issue where we are currently not able to provide an existing ConfigMap containing loki configuration.

...
loki:
  useExternalConfig: true # We do not want the loki-config ConfigMap to be created
  configStorageType: Secret
  externalConfigSecretName: loki-config
...

Templates rendered with this values do not contain theloki-config ConfigMap (like we expect it to be) but we still have the following section in some of the manifests which gives errors when deploying the chart

volumes:
- name: config
  secret:
    secretName: loki-config

To fix this, I took example on the mimir-distributed helm chart as the use case is pretty much the same. As a result, I renamed the loki.existingSecretForConfig property which was confusing to loki.useExternalConfig

Which issue(s) this PR fixes:
N/A

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
  • If the change is worth mentioning in the release notes, add add-to-release-notes label
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR

[CLAassistant]

All committers have signed the CLA.

[Jayclifford345]

Hi @romainDavaze, I know its been a few years is this still a feature you would like to get into Loki. I can see the value in it and apolgies its taken us this long to get to this