Support internal-only load balancer configuration for nginx-ingress

Summary

By default, the NGINX Ingress Controller used with GitLab's Helm chart creates a publicly-facing load balancer. This presents some security risks, and some customers have expressed interest in deploying with only an internal load balancer, and no external LB, for this reason.

We have a configuration available for creating an internal load balancer in addition to the default external one. What we don't have is:

1. An out-of-the box configuration option in the nginx-ingress chart for deploying with only an internal load balancer
2. Documentation on how to deploy nginx-ingress with only an internal load balancer and no external one

This Stack Overflow entry includes an example of what the config can look like: https://stackoverflow.com/a/70349632

Here is the Zendesk ticket (internal only) from a customer requesting this feature.