Serve Jenkins from paas-docker nginx

Summary:
Provision front-end nginx configuration to serve
the continuous deployment Jenkins server cd.nasqueron.org.

As it's the first commit to configure this nginx instance,
we take care of Let's encrypt and proxy configuration.

The specific proxy configuration is fairly standard,
but tweaked to ensure compatibility with Jenkins CLI
in HTTP connection mode.

The docker_containers pillar entry format is switched from a list
to a dictionary, to allow to store metadata like domain or port.

Area for improvement

Ideally, we should serve static files from nginx, but the container
doesn't contain a nginx instance and we don't have access to those
files.

References

• https://jenkins.io/doc/book/managing/cli/
• https://wiki.jenkins.io/display/JENKINS/Running+Jenkins+behind+Nginx

Test Plan: Browse https://cd.nasqueron.org

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1413