Add utility to check if a file is a valid txn_box config file

[rob05c]

Add a utility to check if a file is a valid txn_box config file, and whether it will load successfully when ATS is reloaded.

This will let users with ATS config generation (such as Apache Traffic Control) determine if a txn_box file, potentially created by untrusted or minimally trusted tenants, will cause ATS to fail to reload, so the config generation tools can decide to not set that file for that remap/service.

[v-init-h]

+1 for the linter

(we only rely on yaml lint and ATS reload to check on errors - if any)