Make the server names nginx hash size configurable

And default it to 64, instead of relying on nginx's poor cache line logic. By making it a config, people can set it higher if they need to.

Fixes https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1249

CPUs with smaller cache lines will have slightly worse performance, but we don't actually expect to be run on those cpus anyways. 64 should be fine for the pi2. 32 would have been ideal for a pentium 3. 😉