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.

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1. Changes are OpenShift compatible
2. CRD changes approved
3. Integration tests passed (for non trivial changes)

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1. Code contains useful comments
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3. Documentation added or updated
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[maltesander]

LGTM!