Automatic merge from submit-queue

[kubeadm] pre-flight check hostname to ensure kubelet can launch static pods li…

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**What this PR does / why we need it**: pre-flight check hostname to ensure kubelet can launch static pods like kube-apiserver/kube-controller-manager

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes # 

**Special notes for your reviewer**:

# what is the influence of this issue?

kubelet will not create api server and kcm pod if your hostname is uncorrect. It complain the config files in "/etc/kubernetes/manifests" are invlid.

# how to reproduce this issue?

change your hostname by `hostnamectl set-hostname vm_81_12_centos`. then run `kubeadm init`. you will get this error log from kubelet:

```log
Oct 27 11:12:57 vm_81_12_centos kubelet: I1027 11:12:57.279458    2695 file.go:123] Can't process config file "/etc/kubernetes/manifests/kube-controller-manager.json": invalid pod: [metadata.name: Invalid value: "kube-controller-manager-vm_81_12_centos": must match the regex [a-z0-9]([-a-z0-9]*[a-z0-9])?(\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)* (e.g. 'example.com') spec.nodeName: Invalid value: "vm_81_12_centos": must match the regex [a-z0-9]([-a-z0-9]*[a-z0-9])?(\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)* (e.g. 'example.com')]
```

# where the error comes from in the code?

`pkg/kubelet/config/file.go:144 sourceFile:extractFromDir`

```go
func (s *sourceFile) extractFromDir(name string) ([]*api.Pod, error) {
	dirents, err := filepath.Glob(filepath.Join(name, "[^.]*"))
	if err != nil {
		return nil, fmt.Errorf("glob failed: %v", err)
	}

	pods := make([]*api.Pod, 0)
	if len(dirents) == 0 {
		return pods, nil
	}

	sort.Strings(dirents)
	for _, path := range dirents {
		statInfo, err := os.Stat(path)
		if err != nil {
			glog.V(1).Infof("Can't get metadata for %q: %v", path, err)
			continue
		}

		switch {
		case statInfo.Mode().IsDir():
			glog.V(1).Infof("Not recursing into config path %q", path)
		case statInfo.Mode().IsRegular():
			pod, err := s.extractFromFile(path)
			if err != nil {
-->				glog.V(1).Infof("Can't process config file %q: %v", path, err)
			} else {
				pods = append(pods, pod)
			}
		default:
			glog.V(1).Infof("Config path %q is not a directory or file: %v", path, statInfo.Mode())
		}
	}
	return pods, nil
}
```

# how to fix it?

1. change hostname by `hostnamectl set-hostname <right host name>` or
2. add `hostnameOverride` config. If hostnameOverride is set, then kubelet will use this value instead of system hostname.

**Release note**:
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…ke kube-apiserver/kube-controller-manager and so on.