kubeadm: Set the kubelet --resolv-conf flag conditionally on init

[stealthybox]

What this PR does / why we need it:
kubeadm init detects if systemd-resolved is running and configures the kubelet to use a working resolv.conf.
This patch also removes the warning message prompting manual user action for this configuration.

/area kubeadm
/area kubelet
/area dns
/kind bug
/priority important-soon

/sig cluster-lifecycle
/assign @timothysc

Which issue(s) this PR fixes
Fixes kubernetes/kubeadm#845

Special notes for your reviewer:
See the difference in KUBELET_KUBEADM_ARGS when running with this build and enabling the resolved daemon on Ubuntu 17.10:

root@vagrant:/vagrant/bin# bash << EOF
systemctl start systemd-resolved
./845_kubeadm init |& tail -n5
cat /var/lib/kubelet/kubeadm-flags.env

./845_kubeadm reset --force |& tail -n2
systemctl stop systemd-resolved
echo nameserver 8.8.8.8 > /etc/resolv.conf

./845_kubeadm init |& tail -n5
cat /var/lib/kubelet/kubeadm-flags.env

EOF
You can now join any number of machines by running the following on each node
as root:

  kubeadm join 10.0.2.15:6443 --token 77q84j.0342evur7rrfrwwx --discovery-token-ca-cert-hash sha256:190040f9c3adf8410bc6766dac79f8679870190564e15e8f8d1704fafa03f678

KUBELET_KUBEADM_ARGS=--cgroup-driver=cgroupfs --cni-bin-dir=/opt/cni/bin --cni-conf-dir=/etc/cni/net.d --network-plugin=cni --resolv-conf=/run/systemd/resolve/resolv.conf
I0603 06:49:16.592482   14106 reset.go:276] [reset] deleting contents of config directories: [/etc/kubernetes/manifests /etc/kubernetes/pki]
I0603 06:49:16.592858   14106 reset.go:290] [reset] deleting files: [/etc/kubernetes/admin.conf /etc/kubernetes/kubelet.conf /etc/kubernetes/bootstrap-kubelet.conf /etc/kubernetes/controller-manager.conf /etc/kubernetes/scheduler.conf]
You can now join any number of machines by running the following on each node
as root:

  kubeadm join 10.0.2.15:6443 --token 8mdart.gp67vq3nh9urq4z5 --discovery-token-ca-cert-hash sha256:da6b2e5841546eae134524b045e782f0dd91a6b53becc8d69c15d9eab9c88758

KUBELET_KUBEADM_ARGS=--cgroup-driver=cgroupfs --cni-bin-dir=/opt/cni/bin --cni-conf-dir=/etc/cni/net.d --network-plugin=cni

Release note:

`kubeadm init` detects if systemd-resolved is running and configures the kubelet to use a working resolv.conf.

[dixudx]

/lgtm

[luxas]

/approve

Thanks @stealthybox 💯!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dixudx, luxas, stealthybox

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• cmd/kubeadm/OWNERS [luxas]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[k8s-github-robot]

[MILESTONENOTIFIER] Milestone Pull Request: Up-to-date for process

@dixudx @stealthybox @timothysc

Pull Request Labels

• sig/cluster-lifecycle: Pull Request will be escalated to these SIGs if needed.
• priority/important-soon: Escalate to the pull request owners and SIG owner; move out of milestone after several unsuccessful escalation attempts.
• kind/bug: Fixes a bug discovered during the current release.

Help

• Additional instructions
• Commands for setting labels

[fejta-bot]

/retest
This bot automatically retries jobs that failed/flaked on approved PRs (send feedback to fejta).

Review the full test history for this PR.

Silence the bot with an /lgtm cancel comment for consistent failures.

[timothysc]

I won't block it but perhaps we can do an audit of warnings and checks past freeze.

[timothysc]

IMO - we should still emit a message as a breadcrumb to the user in case anything happens.

[k8s-github-robot]

/test all [submit-queue is verifying that this PR is safe to merge]

[k8s-github-robot]

Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions here.