Support for certificate chains

[damoxc]

There doesn't appear to be a way to include a chain in the resulting certificates. This is proving an issue trying to set up etcd using Vault as an intermediate CA for a pre-existing PKI set up.

It looks like there's the ability to call tls.get_chain() when writing out a certificate so appending it to the resulting certificate shouldn't be too difficult to include. Please do correct me if I'm wrong with this assessment though.