Trigger repository-scoped cgroups on startup - v2

Another attempt at !7094 (merged)

Cgroups managed by Gitaly are nested under a subdirectory of the cgroups mountpoint, configurable by the user as the hierarchy_root option. See https://docs.gitlab.com/ee/administration/gitaly/configure_gitaly.html#configure-repository-cgroups-new-method for more information.

hierarchy_root is expected to exist on the system before Gitaly is started; the administrator is expected to create this subdirectory and chown the appropriate files within the directory to the git user.

While this is fairly simple with a VM setup, it gets more complicated when Gitaly is deployed in a Kubernetes cluster. There will be more risk that permissions aren't configured correctly when the cgroups mountpoint is mounted into the container.

To catch these issues early, we should create a repository-scoped cgroup when the cgroups manager is initialised. This would validate the end-to- end configuration and ensure we don't fail with an opaque error when we later try to execute a git process.

Modify Setup() to create a repository-scoped cgroup with a repository ID of 0. This triggers the creation of the repos-0 subdirectory under hierarchy_root but does not add a process. Creating this subdirectory is sufficient in testing permissions