Add support for shareProcessNamespace

[marcusianlevine]

We are using TLS certificates issued via LetsEncrypt, which means we need to periodically reload the Vault servers to pick up the new certificates.

The chart already supports sidecar containers via server.extraContainers, but in order to issue the SIGHUP signal and trigger a restart, the Pod spec needs shareProcessNamespace: true to be available

Ideally this would just be a flag like server.shareProcessNamespace with a default value of false

I'm happy to open a PR for this feature if the maintainers are open to the idea

[jasonodonnell]

This functionality was added in #174. You can expect this to be in the next release!