Enrollment configuration change request: rename "profile" to "default" and change sub-entry semantics

[mbartosch]

This change request would change semantics, but not logic of the enrollment interface configuration.
The current default "profile" definition in the enrollment configuration is confusing IMO:

profile:
  cert_profile: tls_server
  cert_subject_style: enroll

The "profile" section defines defaults, hence it should be named as such. The subkey "cert_profile" repeats the same name "profile" without actually stating that this is a default. The same is true for "cert_subject_style".

Change request: change semantics to

default:
    profile: tls_server
    subject_style: enroll

This makes it easier to understand that tls_server is the default profile of this interface, and enroll is the default subject_style. It is also possible to add additional defaults not related to profiles.

Suggested implementation strategy to ensure backward compatibility:

• read from "default.profile" and "default.subject_style" node
• if no configuration was read, read from "profile.cert_profile" and "profile.cert_subject_style" respectively as a fallback