Create a /healthz style probe

[davecheney]

Contour should be defining probes so that traffic isn't sent to an instance until it is ready. Similarly, this should support a more graceful drain mechanism when downscaling.

The way I thought about doing this was injecting a default vhost into Envoy pointing to Contour that would provide a /healthz endpoint. The idea being that until both Envoy and Contour were up; hitting the node via IP or internal host name (as the ELB does) would not return healthy until both processes were running.