request for post-install Helm hook

The discussion around the implementation of the `postStart` container lifecycle hook in #315 considered a Helm hook instead, but ended up going in the other direction. I think the Helm hook is still needed:

- `postStart` is run each time the container is started, which means it has to be diligent in making sure it's idempotent;
- `postStart` is run for each container, which in an HA configuration means they either race or they have to coordinate;
- `postStart` is not guaranteed to run after the container is created, so it has to be prepared to wait until Vault is up and running enough that it can do the things you want `postStart` to do.

For unsealing (assuming you don't have access to a secure auto-unseal), this is probably fine, but for initialization and first-time setup of secrets engines, &c, this is really kind of a pain.

The alternative I have in mind is to treat the Vault Helm chart as a dependency and put the `post-install` hook in my parent chart.