Upgrade CockroachDB via Helm

[jseldess]

Our current Helm instructions do not use helm commands to upgrade a CockroachDB cluster. There was a specific reason for not using helm here, but it's been lost to history, it seems.

• This note might touch on why we didn’t just use helm upgrade: https://github.com/helm/charts/tree/master/stable/cockroachdb#upgrading.
• This section also mentions problems with helm upgrade and StatefulSets: https://github.com/helm/charts/tree/master/stable/cockroachdb#scaling

Guidance from @DuskEagle:

I think it will only work if you delete the cluster-init.yaml or cluster-init-secure.yaml before issuing the helm upgrade command.