Provide a Let's encrypt unit

Summary:
Our current goal is to provide an unit to use Let's encrypt certificates
everywhere. For that we create a new letsencrypt unit in the core role.

This change focus to cleanup the repository and consolidate Let's encrypt
artefacts in one place.

We first integrate to this unit the renew service prepared at cd39c567ec4f
after a little cleanup (oneshot mode, use the default service name for timer).
We then move the software requirement here too.

The certbot name is enforced.

Test Plan:

• assert Salt checks for the certbot package
• test the service on Eglide

Reviewers: Sandlayth

Differential Revision: https://devcentral.nasqueron.org/D965