Use StepAbs for all files in the ca.json

[maraino]

Description

Currently, some files in the ca.json, like the certificate templates, can be configured relative to the STEPPATH, for example, if a template file is set in the ca.json to "templates/x509/leaf.tpl" the location of the file would be relative to the STEPPATH, it will be $(step path)/templates/x509/leaf.tpl.

We should make this functionality available across all the files in the ca.json. That way, we can have things like:

{
   "root": "certs/root_ca.crt",
   "crt": "certs/intermediate_ca.crt",
   "key": "secrets/intermeidate_ca_key",
   "...": {}
}

[maraino]

StepAbs is defined in https://github.com/smallstep/cli/blob/779312530411ecf197ecfeef7d9250d1e79e541f/config/config.go#L48-L74

[tashian]

+1 if it is a quick fix, as it would be helpful for NixOS integration