Add the ability to override default registry and set imagePullSecrets

[burmanm]

… for medusa/reaper/stargate

What this PR does:
Adds two new env variables, DEFAULT_REGISTRY which overrides the default registry used to deploy external images and a IMAGE_PULL_SECRETS that allows to set certain imagePullSecrets always.

The Helm chart already had values for these, .image.registryOverride and .imagePullSecrets[]

Running the following command:

helm template charts/k8ssandra-operator --api-versions=cert-manager.io/v1 --set 'imagePullSecrets={a,b,c}' --set image.registryOverride=localhost:5001

Gives us the following deployment:

        - name: DEFAULT_REGISTRY
          value: "localhost:5001"
        - name: IMAGE_PULL_SECRETS
          value: "a,b,c"

Which issue(s) this PR fixes:
Fixes #1280

Checklist

• Changes manually tested
• Automated Tests added/updated
• Documentation added/updated
• CHANGELOG.md updated (not required for documentation PRs)
• CLA Signed: DataStax CLA

[olim7t]

[nitpick] I imagine it will be too late to address this since these are pre-existing values, but naming could have been better:

• image.registry and image.registryOverride: based on names alone it would seem the latter overrides the former, but if I'm following correctly it's more like "registry for the operator image" vs. "registry for the images of the pods created by the operator".
• image.registryOverride and imagePullSecrets are closely related, yet they live at different levels.

[sonarqubecloud]

Quality Gate failed

Failed conditions
4.4% Duplication on New Code (required ≤ 3%)

See analysis details on SonarCloud

[olim7t]

👍 The default is now correctly applied.