refactor: rename CLUSTER_* environment variables to follow Kubernetes naming conventions

[rgolangh]

Background

PR #321 raised the concern that CLUSTER_HOST, CLUSTER_USERNAME, CLUSTER_PASSWORD, and CLUSTER_VERIFY_SSL are too vague as environment variable names.

Rather than using MTV_API_TESTS_CLUSTER_* (which is verbose), we should research and adopt a naming convention consistent with what other Kubernetes/OpenShift projects use for cluster credential environment variables (e.g., KUBECONFIG, KUBE_HOST, etc.).

Requirements

• Research naming conventions used by popular k8s projects for cluster credential env vars
• Choose a prefix/convention that is standard and recognizable
• Rename all occurrences across Python code, documentation, and CI configuration
• Update all docs (README.md, docs/.md, guides/.md) to reflect the new names

Deliverables

• Research k8s ecosystem naming conventions
• Propose naming scheme (comment on this issue before implementing)
• Rename env vars in code (utilities/utils.py, cli/mtv_api_tests/common.py, cli/mtv_api_tests/run.py)
• Update all documentation files
• Update any CI/CD configuration that references these env vars