Remove defunctLabel check due to rewrite of StatefulSets these days, …

[burmanm]

…fix some CRD comments, modify created-by label value to match Kubernetes recommendation

What this PR does: Removes defuctLabel check for managed-by label, changes created-by label value and updates some CRD descriptions.

Which issue(s) this PR fixes: Fixes #354

Checklist

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

[burmanm]

Actually, the created-by fix is wrong. Created-by should not be pushed to CassandraDatacenter, since cass-operator is not creating that resource, only managing it.

[burmanm]

k8ssandra-operator should add the created-by..