Zero-to-jupyterhub-k8s: Move from kube-lego to cert-manager

[yuvipanda]

From @willingc in https://github.com/jupyterhub/zero-to-jupyterhub-k8s/issues/437#issuecomment-361342249:

We should begin looking at cert-manager as kube-lego is deprecated and will not support kubernetes > 1.8. We're currently using Kubernetes 1.8.6.

Here's a guide for migrating to cert-manager:
https://github.com/jetstack/cert-manager/blob/master/docs/tutorials/acme/migrating-from-kube-lego.rst

Possibly related DNS issue jetstack/kube-lego#297

We should switch to cert-manager for 0.7, and hopefully do so in a way that doesn't interrupt user service!

[yuvipanda]

Bumping off v0.7, don't think we'll do this now.

[clkao]

cert-manager with ingressShim actually works pretty well out of the box, if our ingress has kubernetes.io/tls-acme defined. I guess we can add this to the doc at least, and see if we prefer people to run their own cert-manager or include it in this chart like lego.

[consideRatio]

@clkao we should make it the default, kube-lego have been deprecated in favor of cert-manager for a while and it sais they only support k8s 1.8.

If you can help us get there I'd be very happy! I'm not confident at all regarding these things.

[clkao]

I've just tested ingressShim-based cert-manager. Happy to help later this week.

[manics]

Fixed by Traefik! https://github.com/jupyterhub/zero-to-jupyterhub-k8s/pull/1539