Add values option to override name of the pods in stateful set

[aronneagu]

Add a values option (.Values.server.nameOverride) that allows to change just the name of the vault pods

The fullnameOverride does not fit the bill as that changes the name for all vault related k8s resources

[hashicorp-cla]

Thank you for your submission! We require that all contributors sign our Contributor License Agreement ("CLA") before we can accept the contribution. Read and sign the agreement

Learn more about why HashiCorp requires a CLA and what the CLA includes

---

Aron Neagu seems not to be a GitHub user.
You need a GitHub account to be able to sign the CLA. If you already have a GitHub account, please add the email address used for this commit to your account.

_{Have you signed the CLA already but the status is still pending? Recheck it.}