Reduce amount of code in pkg/k8sutil package

[invidian]

Generally util packages are considered a smell and from what I see, there is actually plenty of code which could be moved around to either operator or agent package, so long we can keep it testable.

$ git grep k8sutil\\. | cut -d '(' -f1 | perl -ne '/(.*\/.*\/).*\.go:.*(k8sutil\..*)/ && print "$1 $2\n";' | sort -u | column -t
cmd/update-agent/     k8sutil.GetClient
cmd/update-operator/  k8sutil.GetClient
pkg/agent/            k8sutil.FilterPods
pkg/agent/            k8sutil.GetNodeRetry
pkg/agent/            k8sutil.GetPodsForDeletion
pkg/agent/            k8sutil.SetNodeAnnotations
pkg/agent/            k8sutil.SetNodeAnnotationsLabels
pkg/agent/            k8sutil.SetNodeLabels
pkg/agent/            k8sutil.Unschedulable
pkg/k8sutil/          k8sutil.UpdateNodeRetry
pkg/operator/         k8sutil.FilterContainerLinuxNodes
pkg/operator/         k8sutil.FilterNodesByAnnotation
pkg/operator/         k8sutil.FilterNodesByRequirement
pkg/operator/         k8sutil.NewRequirementOrDie
pkg/operator/         k8sutil.SetNodeLabels
pkg/operator/         k8sutil.UpdateNodeRetry