feat: support custom images in integration tests

[razvan]

Description

This was done to be able to test : https://github.com/stackabletech/docker-images/pull/663 as seen in the test-definition.yaml comments.

:warning: CI: https://ci.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/nifi-operator-it-custom/55/

:green_circle: Fixing and rerunning the failed CI tests individually:

$ ./scripts/run-tests --skip-release --test-suite openshift --skip-delete --test upgrade
...
--- PASS: kuttl (405.61s)
    --- PASS: kuttl/harness (0.00s)
        --- PASS: kuttl/harness/upgrade_zookeeper-latest-3.9.2_nifi_old-1.21.0_nifi_new-1.25.0,docker.stackable.tech_sandbox_nifi_1.25.0-stackable0.0.0-dev_openshift-true (394.15s)
PASS

:green_circle: And

$ ./scripts/run-tests --skip-release --test-suite openshift --skip-delete --test cluster_operation
...
--- PASS: kuttl (346.00s)
    --- PASS: kuttl/harness (0.00s)
        --- PASS: kuttl/harness/cluster_operation_zookeeper-latest-3.9.2_nifi-latest-1.25.0,docker.stackable.tech_sandbox_nifi_1.25.0-stackable0.0.0-dev_openshift-true (334.52s)
PASS

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
- [x] Changes are OpenShift compatible
- [x] CRD changes approved
- [x] CRD documentation for all fields, following the [style guide](https://docs.stackable.tech/home/nightly/contributor/docs-style-guide).
- [x] Helm chart can be installed and deployed operator works
- [x] Integration tests passed (for non trivial changes)
- [x] Changes need to be "offline" compatible

# Reviewer
- [ ] Code contains useful comments
- [ ] Code contains useful logging statements
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated. Follows the [style guide](https://docs.stackable.tech/home/nightly/contributor/docs-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
- [ ] [Roadmap](https://github.com/orgs/stackabletech/projects/25/views/1) has been updated