dice-cert-tmpl: replace openssl crimes w/ purpose built code

[flihp]

This is a good candidate for using pki-playground to generate the certificate templates we ship in hubris. pki-playground will generate the certs for us, then we'll need a bit of code to strip the fields we want left blank and find the required offsets. The end goal is to integrate this into a build.rs file in hubris that generates these cert templates as part of the build process.