Add KUBERNETES_CLUSTER_DOMAIN to operator ENV variable docs

[maltesander]

In https://github.com/stackabletech/issues/issues/436 a new ENV variable KUBERNETES_CLUSTER_DOMAIN was introduced for our operators.

This should be documented (and was forgotten) as well in the ENV VAR section of the operators.

Proposed snippet:

== KUBERNETES_CLUSTER_DOMAIN

*Default value*: cluster.local

*Required*: false

*Multiple values*: false

This instructs the operator, which value it should use for the Kubernetes `clusterDomain` setting.
Make sure to keep this in sync with whatever setting your cluster uses.
Please see the documentation xref:guides:kubernetes-cluster-domain.adoc[on configuring the Kubernetes cluster domain] for more information on this feature.

[source]
----
export KUBERNETES_CLUSTER_DOMAIN=mycluster.local
cargo run -- run
----

or via docker:

[source]
----
docker run \
--name <product>-operator \
--network host \
--env KUBECONFIG=/home/stackable/.kube/config \
--env KUBERNETES_CLUSTER_DOMAIN=mycluster.local \
--mount type=bind,source="$HOME/.kube/config",target="/home/stackable/.kube/config" \
docker.stackable.tech/stackable/<product>-operator:latest
----

### Products
- [ ] https://github.com/stackabletech/airflow-operator/pull/537
- [ ] https://github.com/stackabletech/druid-operator/pull/648
- [ ] https://github.com/stackabletech/hbase-operator/pull/585
- [ ] https://github.com/stackabletech/hdfs-operator/pull/604
- [ ] https://github.com/stackabletech/hive-operator/pull/535
- [ ] https://github.com/stackabletech/kafka-operator/pull/786
- [ ] https://github.com/stackabletech/nifi-operator/pull/709
- [ ] https://github.com/stackabletech/spark-k8s-operator/pull/490
- [ ] https://github.com/stackabletech/superset-operator/pull/561
- [ ] https://github.com/stackabletech/trino-operator/pull/665
- [ ] https://github.com/stackabletech/zookeeper-operator/pull/882
- [ ] https://github.com/stackabletech/opa-operator/pull/648
- [ ] Commons
- [ ] Listener
- [ ] Secret
- [ ] Hello World

[lfrancke]

@maltesander I created PRs for everything I could find.

Commons, listener, secret and hello world don't seem to have that variable, is that possible?

[maltesander]

@maltesander I created PRs for everything I could find.

Commons, listener, secret and hello world don't seem to have that variable, is that possible?

This was the rollout https://github.com/stackabletech/issues/issues/652. It should be in everything except the EDC op which we decided to punt on.

[lfrancke]

Okay, they don't have any page for the environment variables at all yet. I'll see if I can put up PRs for this tomorrow. But if you get to it before me feel free to do so.