Store kubernetes clientset when waiting for apiserver

While looking at pprof data, I noticed that the line:

```go
client, err = k.client(k8s)
```

takes around .9 seconds to complete. Previously, we were calling it on
every iteration of the waitFor loop. Instead, if we store the clienset once
we have it, we should be able to speed up this function.

During local testing, this optimization reduced time spent waiting for
the apiserver by 13 seconds on average.

pkg/minikube/bootstrapper/kubeadm/kubeadm.go

@@ -509,6 +509,7 @@ func (k *Bootstrapper) waitForAPIServer(k8s config.KubernetesConfig) error {
 	}
 
 	glog.Infof("Waiting for apiserver to port healthy status ...")
+	var client *kubernetes.Clientset
 	f := func() (bool, error) {
 		status, err := k.GetAPIServerStatus(net.ParseIP(k8s.NodeIP), k8s.NodePort)
 		glog.Infof("apiserver status: %s, err: %v", status, err)
@@ -520,10 +521,13 @@ func (k *Bootstrapper) waitForAPIServer(k8s config.KubernetesConfig) error {
 			return false, nil
 		}
 		// Make sure apiserver pod is retrievable
-		client, err := k.client(k8s)
-		if err != nil {
-			glog.Warningf("get kubernetes client: %v", err)
-			return false, nil
+		if client == nil {
+			// We only want to get the clientset once, because this line takes ~1 second to complete
+			client, err = k.client(k8s)
+			if err != nil {
+				glog.Warningf("get kubernetes client: %v", err)
+				return false, nil
+			}
 		}
 
 		_, err = client.CoreV1().Pods("kube-system").Get("kube-apiserver-minikube", metav1.GetOptions{})