Automatically configure unicorn to spawn as much worker processes as
there are CPUs on the system by default.

GitLab omnibus pre-hardcodes this value default to 2 (which we copied)
and then also tweaks it this way in active code

    https://gitlab.com/gitlab-org/omnibus-gitlab/blob/8eda0933/files/gitlab-cookbooks/gitlab/attributes/default.rb#L230

which we also do here.

/cc @kazuhiko, @jerome