v2.1: Validate certificate vs private_key

[johnnyshields]

Currently there is no validation that certificate actually matches private_key. It would be good to add this, because it may cause headaches for users to debug this if for some reason their keys are out-of-sync.

[pitbulk]

We can add a method to check if a pair of cert and private_key are related. Then extend validate_sp_certs_params! method to use it

[johnnyshields]

Right, I think this is only supported on more recent versions of OpenSSL. I will check.

[pitbulk]

If that's the case, we can simply check whether the OpenSSL method is available and, based on that, execute the extra check. Rather than forcing people to have a specific OpenSSL version.