Allow nginx to read /.well-known/acme-challenge

Summary:
Certbot write files in /var/letsencrypt-auto/.well-known/acme-challenge
to allow Let's Encrypt server to verify the certificate request comes
from an authorized source.

Fixes T2051.

Test Plan:

• echo "It works!" > /var/letsencrypt-auto/.well-known/acme-challenge/qa.txt
• Browse https://builds.nasqueron.org/.well-known/acme-challenge/qa.txt

Reviewers: ledesillusionniste

Reviewed By: ledesillusionniste

Subscribers: DorianWinty

Maniphest Tasks: T2051

Differential Revision: https://devcentral.nasqueron.org/D3501