Check public.yml for syntax errors

[jamesmontalvo3]

When public.yml is loaded by the set-vars role, it is done with failed_when: False in order to allow public.yml to not exist. This is required for bootstrapping a new server. However, this also allows public.yml to have hidden syntax errors which can cause unexpected issues on a server. Syntax-check public.yml and throw an error during deploy if there are issues.

https://github.com/enterprisemediawiki/meza/blob/619bea6658914e8e77bfda0691fa3c03d30c1cdc/src/roles/set-vars/tasks/main.yml#L12-L16