[feature request] ability to use pki sign endpoint in vault

[n-marton]

It would be really nice if consul-template would support not just pki issue but also pki sign endpoint in vault.

When one has a system with a lots of clients and a low cert ttl, the amount of private key generation can cause quite a heavy load on the vault side in the case of pki issue, however if one uses the pki sign endpoint the private key generation remains on the client side, I made a PR about this some time ago, could this get a bit of attention?

#1905