Create a new scenario to run the tests with Praefect

We need a new scenario to configure GitLab to use Praefect instead of Gitaly directly. It will need to:

This is part of the HA Gitaly work: &2006 (closed)

Docs: https://docs.gitlab.com/ee/administration/gitaly/praefect.html

For the sake of preparation and testing, GDK has some rake tasks that help with setting it up to use Praefect: https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/lib/tasks/praefect.rake (but gitlab-qa uses omnibus images, so ultimately that's what we'll need to use)

As Praefect is intended to be transparent, I don't believe it's necessary to run the new test scenario with every MR. Therefore, I'll add it to the nightly pipeline (https://gitlab.com/gitlab-org/quality/nightly/merge_requests/42), but not to gitlab-qa (so it won't run as part of package-and-qa).

Staging is also now using Praefect as the default storage, so the smoke tests will provide relatively quick feedback (within an hour) if something goes wrong.