[node-agent] Introduce hostname-check controller

[rfranzke]

How to categorize this PR?

/area usability
/kind bug

What this PR does / why we need it:
With this PR, a new hostname-check controller is introduced. It periodically (each 30s) checks if the hostname of the node gardener-node-agent runs on has changed compared to what it has read during its start-up. If a change is detected, it calls the cancel function of the root context, leading to a termination and an eventual restart of the gardener-node-agent systemd unit.

Motivation: We have seen cases where the hostname has changed for some nodes. Without this, gardener-node-agent will stuck forever in

gardener/pkg/nodeagent/controller/operatingsystemconfig/reconciler.go

Lines 155 to 158 in 76704c3

	if node == nil {
	log.Info("Waiting for Node to get registered by kubelet, requeuing")
	return reconcile.Result{RequeueAfter: 5 * time.Second}, nil
	}

until it gets restarted. In addition, the hostname is used to compute a label selector predicate here:

gardener/pkg/nodeagent/controller/add.go

Lines 36 to 39 in 76704c3

	nodePredicate, err := predicate.LabelSelectorPredicate(metav1.LabelSelector{MatchLabels: map[string]string{corev1.LabelHostname: hostName}})
	if err != nil {
	return fmt.Errorf("failed computing label selector predicate for node: %w", err)
	}

Which issue(s) this PR fixes:
Part of #8023
Follow-up of #9114

Special notes for your reviewer:
/cc @oliver-goetz @ialidzhikov

Release note:

`gardener-node-agent` now terminates itself (leading to a restart of its `systemd` unit) in case it determines that the hostname of its node has changed.

[rfranzke]

/cherry-pick release-v1.89

[gardener-ci-robot]

@rfranzke: once the present PR merges, I will cherry-pick it on top of release-v1.89 in a new PR and assign it to you.

In response to this:

/cherry-pick release-v1.89

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[rfranzke]

/cherry-pick release-v1.88

[rfranzke]

/cherry-pick release-v1.87

[gardener-ci-robot]

@rfranzke: once the present PR merges, I will cherry-pick it on top of release-v1.88 in a new PR and assign it to you.

In response to this:

/cherry-pick release-v1.88

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[gardener-ci-robot]

@rfranzke: once the present PR merges, I will cherry-pick it on top of release-v1.87 in a new PR and assign it to you.

In response to this:

/cherry-pick release-v1.87

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[oliver-goetz]

awesome 🚀
/lgtm
/approve

[gardener-prow]

LGTM label has been added.

Git tree hash: 16191e75c27071003c52c203b1ad39cd721a020d

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: oliver-goetz

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[gardener-ci-robot]

@rfranzke: new pull request created: #9286

In response to this:

/cherry-pick release-v1.89

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[gardener-ci-robot]

@rfranzke: new pull request created: #9287

In response to this:

/cherry-pick release-v1.88

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[gardener-ci-robot]

@rfranzke: new pull request created: #9288

In response to this:

/cherry-pick release-v1.87

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.