Prepare for Gitaly absorbing gitlab-shell hooks

Fixes #211 (closed).

This should be merged before gitaly!1130 (merged) so that by the time a new gitaly CNG image is built, we have this fix in place in our CI.

The change here is that we use a new environment variable (introduced in gitaly!1130 (merged)) to configure Gitaly to not use Git hooks at all. This is a requirement for our tests, because the Git hooks try to make HTTP requests to gitlab-rails, and we don't boot gitlab-rails during the gitlab-workhorse tests.