Allow us to explicitly state where keys, certs, csr and ca files are stored

This moves the logic that binds paths out of CertificateRequest and into the command line tooling.

Further, it adds the ability to set exact paths for the separate requested parts, making it possible to better create secrets in kubernetes.

Rationale: in k8s and on devices, certificates need an generated client_id but an well known path for each.

Issue: #70