Make it easier to use a custom image in tests

[lfrancke]

Description

Make it easier to use a custom image in tests. This follows the best practice from other operators.

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

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1. Integration tests passed (for non trivial changes)

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1. Changelog updated

[adwk67]

Successfully tested:

nightly, with only the custom image:

--- PASS: kuttl (2872.22s)
    --- PASS: kuttl/harness (0.00s)
        --- PASS: kuttl/harness/kerberos_hadoop-latest-3.4.0,docker.stackable.tech_sandbox_hadoop_3.4.0-stackable0.0.0-dev_zookeeper-latest-3.9.2_krb5-1.21.1_kerberos-realm-CLUSTER.LOCAL_kerberos-backend-mit_openshift-false (503.86s)
        --- PASS: kuttl/harness/smoke_hadoop-3.4.0,docker.stackable.tech_sandbox_hadoop_3.4.0-stackable0.0.0-dev_zookeeper-3.9.2_zookeeper-latest-3.9.2_number-of-datanodes-2_datanode-pvcs-2hdd-1ssd_listener-class-cluster-internal_openshift-false (265.98s)
        --- PASS: kuttl/harness/orphaned-resources_hadoop-latest-3.4.0,docker.stackable.tech_sandbox_hadoop_3.4.0-stackable0.0.0-dev_zookeeper-latest-3.9.2_openshift-false (158.33s)
        --- PASS: kuttl/harness/logging_hadoop-3.4.0,docker.stackable.tech_sandbox_hadoop_3.4.0-stackable0.0.0-dev_zookeeper-latest-3.9.2_openshift-false (541.16s)
        --- PASS: kuttl/harness/smoke_hadoop-3.4.0,docker.stackable.tech_sandbox_hadoop_3.4.0-stackable0.0.0-dev_zookeeper-3.9.2_zookeeper-latest-3.9.2_number-of-datanodes-2_datanode-pvcs-2hdd-1ssd_listener-class-external-unstable_openshift-false (221.40s)
        --- PASS: kuttl/harness/cluster-operation_hadoop-latest-3.4.0,docker.stackable.tech_sandbox_hadoop_3.4.0-stackable0.0.0-dev_zookeeper-latest-3.9.2_openshift-false (331.24s)
        --- PASS: kuttl/harness/profiling_hadoop-3.4.0,docker.stackable.tech_sandbox_hadoop_3.4.0-stackable0.0.0-dev_zookeeper-latest-3.9.2_openshift-false (140.37s)
        --- PASS: kuttl/harness/topology-provider_hadoop-latest-3.4.0,docker.stackable.tech_sandbox_hadoop_3.4.0-stackable0.0.0-dev_zookeeper-latest-3.9.2_krb5-1.21.1_kerberos-backend-mit_openshift-false (285.88s)
        --- PASS: kuttl/harness/kerberos_hadoop-latest-3.4.0,docker.stackable.tech_sandbox_hadoop_3.4.0-stackable0.0.0-dev_zookeeper-latest-3.9.2_krb5-1.21.1_kerberos-realm-PROD.MYCORP_kerberos-backend-mit_openshift-false (423.65s)
PASS

Smoke tests, mixed custom/non-custom images:

--- PASS: kuttl (337.05s)
    --- PASS: kuttl/harness (0.00s)
        --- PASS: kuttl/harness/smoke_hadoop-3.3.6_zookeeper-3.9.2_zookeeper-latest-3.9.2_number-of-datanodes-1_datanode-pvcs-2hdd-1ssd_listener-class-cluster-internal_openshift-false (207.44s)
        --- PASS: kuttl/harness/smoke_hadoop-3.4.0,docker.stackable.tech_sandbox_hadoop_3.4.0-stackable0.0.0-dev_zookeeper-3.9.2_zookeeper-latest-3.9.2_number-of-datanodes-1_datanode-pvcs-2hdd-1ssd_listener-class-cluster-internal_openshift-false (129.27s)
PASS

[adwk67]

LGTM!