Include Praefect in the canary stack

Currently we have canary web/api/git and file nodes. With the addition of Praefect as an intermediary between the former and the later, we should also have a canary Praefect node, to ensure all components of a canary deploy can be properly tested and accounted for.

Steaming from this discussion

/cc @albertoramos @stanhu @jacobvosmaer-gitlab