fix: Revert default cluster domain to 'cluster.local'

[dervoeti]

Description

Revert the change of the default cluster domain introduced by #939.

Reasoning: We came across some issues with TLS certificates and Kerberos keytabs and decided to make this behavior (FQDN domains for services/pods) opt-in for now. We still wanted to keep experimental support for it, but only if a user opts in by explicitly setting KUBERNETES_CLUSTER_DOMAIN to a domain with a trailing dot.

I tested this change with Zookeeper, both with and without KUBERNETES_CLUSTER_DOMAIN set, works as expected.

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
- [ ] Changes are OpenShift compatible
- [ ] CRD changes approved
- [ ] Integration tests passed (for non trivial changes)

# Reviewer
- [ ] Code contains useful comments
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated
- [ ] 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

[maltesander]

LGTM!