Support "private" OpenWhisk deployment

I'm in a situation where I'd like to deploy OpenWhisk for use by other components within a cluster. In particular, I don't really want to expose anything to the public internet, which seems to be impossible with the current setup.

The current options (excluding OpenShift) for `whisk.ingress.type` are `LoadBalancer`, `NodePort`, and `Standard` (which is essentially just a `ClusterIP` service with a "frontend" ingress).

Could we add an option for a `None` ingress (which just creates a ClusterIP service and doesn't create a frontend) or an option to disable the frontdoor ingress (something like `whisk.ingress.frontDoorEnabled` as a boolean)? This might also be useful in setups where the ingress is more complicated and so you want to manage it outside of this chart (e.g., using cloud-provider managed certificates).

I'd be happy to make a PR to add an option to disable frontdoor if that sounds okay (afaict it would be three lines of code plus some documentation).

Thanks for all the awesome work on this!

Start by tracing the chart's handling of `whisk.ingress.type` and the frontend ingress, then review the existing documentation for ingress options. Done means the chart can deploy privately with only a ClusterIP service and no frontend, while preserving the current ingress modes and documenting the new option.

Written by the indexing model from the issue text.