This script allows to check on a machine the nginx (or any other
web server) configuration: do we serve correctly the acme challenge
at http://<domain>/.well-known/acme-challenge?
It checks the page is 200 and with the challenge content. If not,
it prints the domain, the error code (or "DOES NOT MATCH" if 200)
to stdout.
It's written in Python 2, not 3, so it runs without any dependency
on Dwellers, our Docker host under CentOS 7, where Python 3 isn't
installed.
Fixes T854.