Install Let's encrypt client as a Docker container

Summary:
The Let's encrypt client certbot is installed according two methods:

• on regular servers, as a distro/OS package
• on Docker engines, as a container and a wrapper script

This change takes care of the second method.

The certbot/certbot image is used (previously, the letsencrypt
image on quay.org was used), as officially maintained by EFF.

A certbot wrapper command runs the container with the correct
volumes mounted to be able to generate or renew certificates.

Test Plan:

• salt equatower state.apply roles/paas-docker/docker/images
• salt equatower state.apply roles/paas-docker/wrappers
• certbot <some commands>

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1422