Vault client certificates aren't reread on reload

Specifically, the certificate used/referred to in the `vault.cert_file` and `vault.key_file` sections of the nomad config. The other certs are reloaded/reread on SIGHUP, but not that one.

It's what I've observed experimentally but it would take a bit of work to set this up for me to be able to log and demonstrate it. My understanding is that nomad doesn't reload every config value so this isn't something I'd have to demonstrate/prove deeply, but I can if the symptoms sound surprising.

Trace the Nomad configuration handling for vault.cert_file and vault.key_file, then compare it with the other certificate paths during SIGHUP reload. Reproduce or verify the reported behavior and update the reload path so those certificates are reread on SIGHUP; done means renewed certificates are used after reload.

Written by the indexing model from the issue text.