consider opening HTTP/HTTPS listeners by default

[skriss]

Please describe the problem you have
[A clear, concise, description of the problem you are facing. What is the problem that feature X would solve for you?]

Currently, Contour does not open Envoy listeners until there is a vhost/route configured for the relevant protocol. This has caused some challenges with health-checking of LoadBalancers. The maintainers have discussed revisiting this decision and possibly always opening the ports/listeners on startup, regardless of whether there are routes.

xref. #389