Envoy v1.11: Investigate using /ready endpoint for probes

[stevesloka]

Describe the solution you'd like
Mentioned in #1242, Envoy v1.11.0 introduces a new /ready admin endpoint which improves readiness probes, this might be a nice thing to change for Contour 0.15 too, maybe even improving smoother Envoy rollouts as discussed on #1245

// cc @rochacon

[stevesloka]

The only nice thing about the way we expose today is that we don't need to expose the entire admin site to a port for the kubelet to check against. So let's keep that in mind when this is investigated such that we don't accidentally expose that admin UI.

[rochacon]

I've been testing Contour with Envoy 1.11 to evaluate the use of /ready and trim down the steps discussed at #145. It works pretty similar to the current /healthz endpoint, with the addition of a response body with the current state of the server (e.g. LIVE / DRAINING).

Contour requires this patch to passthrough /ready to the admin in the restricted listener (port 8002): master...rochacon:enable-ready-endpoint

I believe we should add the above patch to allow the use of /ready even though it won't change the current probe behavior it allows this more specific endpoint to be used. If you agree I'll open the PR for it.

[stevesloka]

@rochacon yup I think switching over to this endpoint is fine, happy to take a PR!

[davecheney]

@rochacon could you send that PR please.