Add KIND_CLUSTER make variable to allow targeting different kind

[burmanm]

…ters

What this PR does: One can now override KIND_CLUSTER parameter in the make, such as with:

make KIND_CLUSTER=k8ssandra-0 docker-kind

That would load (and build) the newest cass-operator to the kind cluster named k8ssandra-0

Which issue(s) this PR fixes: Fixes #

Checklist

• [x] Changes manually tested
• [ ] Automated Tests added/updated
• [ ] Documentation added/updated
• [ ] CHANGELOG.md updated (not required for documentation PRs)
• [x] CLA Signed: DataStax CLA