Limit gitaly memory usage on nfs servers

This issue tracks the progress of implementing cgroups memory limits for gitaly.

• Create a way to manage memory cgroups on nfs servers (https://gitlab.com/gitlab-cookbooks/gitlab_cgroups_gitaly)
• Enable on staging server with 50% RAM hard limit (https://dev.gitlab.org/cookbooks/chef-repo/merge_requests/1035).
• Run some tests, make sure it behaves when:
  • gitaly is restarted
  • gitaly anything is killed if eats to much ram
• Enable on one production server (bump limit to 90% set to 32G on nfs08 so that system have more than enough fs cache and is stable)
• Enable on all production servers

cc @gl-infra @bkc @andrewn This issue closes https://gitlab.com/gitlab-com/infrastructure/issues/2364 and replaces it for further steps, as the investigation in done.