MEDIUM: ssl: Support certificate chaining for certificate generation

MEDIUM: ssl: Support certificate chaining for certificate generation

haproxy supports generating SSL certificates based on SNI using a provided
CA signing certificate. Because CA certificates may be signed by multiple
CAs, in some scenarios, it is neccesary for the server to attach the trust chain
in addition to the generated certificate.

The following patch adds the ability to serve the entire trust chain with
the generated certificate. The chain is loaded from the provided
`ca-sign-file` PEM file.