cgroups: use stable path

Instead of creating a new directory for cgroups under gitaly- each time Gitaly starts up, use the hierarchy root. Unless we add a cleanup mechanism that cleans up each of the old cgroup directories after a restart, cadvisor will continue to parse those directories in order to obtain metrics. This ends up being wasteful. In addition, it ends up multiplying the cardinality of the metrics each time.

If we simply reuse the same directory, these problems go away, and there is not a big advantage to using separate cgroup directories per restart.

Changelog: changed