[kong] define KONG_PORT_MAPS for the proxy service

[mflendrich]

What this PR does / why we need it:

Defines KONG_PORT_MAPS for the proxy service.
Before this PR, a request coming to the proxy service on port 80 would get X-Forwarded-Port: 8000 (and equivalent for https).
After this PR, a request coming to the proxy service on port 80 would get X-Forwarded-Port: 80 (and equivalent for https).

Makes use of the new feature introduced in Kong/kong#5861.
This PR makes a fix equivalent to Kong/kubernetes-ingress-controller#691, but for a different deployment method.

Which issue this PR fixes

fixes #168

Special notes for your reviewer:

Note that KONG_PORT_MAPS get defined only for the proxy service.

Checklist

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• PR is based off the current tip of the next branch and targets next, not master
• Title of the PR and commit headers start with chart name (e.g. [kong])