Changing registry port is not obvious or documented

In gitlab-org/gitlab-ce#19817, it's apparent that it's really not obvious how to change the default container registry port from 5000, nor is it documented how.

This also came up with a customer. ZenDesk: https://gitlab.zendesk.com/agent/tickets/30951

If you look at the gitlab.rb comments, you might try any of the following options:

registry_api_url 'http://localhost:5001'
gitlab_rails['registry_api_url'] = "http://localhost:5001"
gitlab_registry['internal_host'] = "http://localhost:5001"

None of those are correct. You have to look through the code to understand the correct change is:

registry['registry_http_addr'] = "localhost:5001"

How can we make this easier? Should we hide these derived values and document the above one clearly?

/cc: @axil, @marin