Allow to delete Let's Encrypt certificate

Summary:
In some cases, a Let's Encrypt certificate managed by Certbot
needs to be regenerated from scratch. As there are several files
to delete, it's convenient to automate the process.

Test Plan: Tested on web-001 with delete-certbot-certificate www.nasqueron.org

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3430