WIP: Create design document for productionizing consul

This creates the document for: https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6132

In keeping with the philosophy of making things public while they are embarassingly incomplete, here is the beginning of a working document for designing how we will use consul clusters in the gitlab.com infrastructure.

Since everyone seems to be very passionate and opinionated about a specific part of this design, I'd encourage everyone to clone this branch and push commits to it, rather than just adding comments.

This document is going to be a group effort. cc/ @gitlab-com/gl-infra