Revise Unicorn memory limits

We have some ongoing experiments with Unicorn memory limits on gitlab.com (please let me know if this is a duplicate issue). I am creating this issue as a reminder that we need to:

• update Omnibus default memory limits and its worker count formula in attributes/default.rb
• update gitlab-ce config.ru memory limit defaults
• update the RAM requirements in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/install/requirements.md#memory

This issue replaces https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1883 (a private 1-year old stalled MR).

cc @pcarranza @marin