Allow overriding embedded image references for testing

[tnozicka]

We should allow overriding the embedded image references to e.g. Grafana or Prometheus for testing purposes. ScyllaOperatorConfig seems like a fitting place.

For production purposes, we need to rely on exactly the same content of the referenced images to be present to avoid incompatibilities breakages so this should only be adjusted for testing purposes. All the fields should be behind unsupported prefix.

Given the components used in the stack can change, all such fields should be optional.

[tnozicka]

done in #2081