Update nginx-ingress chart to utilizing built-in Prometheus metrics

With the release of nginx-ingress 0.9.0 betas, internal support for Prometheus metrics has been added. See: https://github.com/kubernetes/ingress-nginx/issues/72

This requires a few minor things:

• enable-vts-status: true
• And to scrape :10254/metrics

However it looks like the nginx-ingress Helm chart is still utilizing a third party side-car container to scrape the VTS metrics and act as the Prometheus exporter.

• See statsExporter fields in the chart

We should update the chart to utilize the new built-in metrics.