Let's encrypt web server configuration checker

Summary:
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.

Test Plan:
Linters: pep8, flake8

Test run:

$ ./check-letsencrypt-certificates
cloud.dereckson.be 404
commons.nasqueron.org 403
$ bash
$ time ./check-letsencrypt-certificates
[…]
real    0m0.733s
user    0m0.300s
sys     0m0.215s

Reviewers: Sandlayth

Maniphest Tasks: T854

Differential Revision: http://devcentral.nasqueron.org/D396