Mostly by using the "extract method" pattern.

Here was the report (from gocyclo), before:

16 kubeadm (*KubeadmBootstrapper).UpdateCluster pkg/minikube/bootstrapper/kubeadm/kubeadm.go:374:1
16 cluster StartHost pkg/minikube/cluster/cluster.go:64:1
16 tunnel setupRoute pkg/minikube/tunnel/tunnel.go:135:1
16 provision configureAuth pkg/provision/buildroot.go:218:1
21 util setElement pkg/util/config.go:50:1