Set the kubelet `--resolv-conf` flag conditionally at kubeadm init time

[luxas]

When kubernetes/kubernetes#63887 merges, we should set the kubelet's --resolv-conf flag to /run/systemd/resolve/resolv.conf conditionally at kubeadm init time, in cmd/kubeadm/app/phases/kubelet/flags.go.

This instead of doing it statically in the deb/rpm packages.
cc @randomvariable @detiber @stealthybox @kad
xref related issue: #822
xref related PR: kubernetes/kubernetes#63632

[luxas]

@randomvariable has volunteered to start working on this in a day or two. Please let us know here when you do 👍

[randomvariable]

Was going to pick this up, but notice you have #847, but we also want run time detection, or sensible defaulting for newer Ubuntu's don't we?

[luxas]

@randomvariable This should be done automatically. If systemd-resolved is used, the kubelet's --resolv-conf argument should be set by kubeadm so we don't break anyone. #847 basically means the user could do it him/herself, and even override kubeadm's defaulting for --resolv-conf, but doesn't mean we should automatically set this flag. Does that make sense to you?

[stealthybox]

/assign

[stealthybox]

We plan to get this into 1.11 before the weekend.
-- discussed in Kubeadm Office Hours - Wed May 30