Webservice: convert puma.rb/unicorn.rb to pull from ENV

Move puma.rb and unicorn.rb from gitlab-webservice to pull their configuration items from environment variables. This allows us to stop maintaining multiple copies between codebases (gitlab, Omnibus, CNG, charts).

Combining ENV with ||=, we provide sane defaults based on the Rails codebase, and the configurations currently generated by the charts.

These changes do not prevent the overiding of these files via mounts, whether in Docker or Kubernetes. This means it is not a breaking change to the charts themselves. When the charts make the change to not provide an overridden configuation, that will be the breaking change.

Part of gitlab-org/charts/gitlab#1419 (closed)

Relates to gitlab-org/charts/gitlab#1419 (closed)