config: set default length only if password policy is missing

[calvn]

Overview

This PR fixes an issue in the config endpoint where the default length value is always set regardless of whether password_policy is provided. This leads to configuration errors unless length=0 is explicitly provided. password_policy and length are mutually exclusive, with preferred on password_policy, so the engine should understand that the value is ignored and not necessary if password_policy is provided.

vault write ad/config \
    binddn=vagrant \
    bindpass=vagrant \
    url=ldaps://127.0.0.1 \
    userdn='dc=Marti,dc=com' \
    password_policy=example

Error writing data to ad/config: Error making API request.

URL: PUT http://127.0.0.1:8200/v1/ad/config
Code: 500. Errors:

* 1 error occurred:
* cannot set password_policy and either length or formatter

Design of Change

We now parse the password_policy first, and fallback to fetching length's default value if both the length and password_policy is absent. The change should be backwards compatible.

Contributor Checklist

• Add relevant docs to upstream Vault repository, or sufficient reasoning why docs won’t be added yet
  Note: Docs already describes this behavior, but the engine was not behaving as expected.
• Backwards compatible

[austingebauer]

LGTM