Switch language name 'zh' to 'zh-cn'

This is the first step to rename 'zh' to 'zh-cn'. There are several reasons why we rename the language name.

- The upstream docsy theme changed the language name, leading to many warnings during site build;
  The side-effect is that the i18n strings are no longer working.
- We believe renaming the language is the right thing to do, because this move can make room for other variants of Chinese language, such as 'zh-tw', 'zh-sg' etc.

There would be several follow-ups to this PR, such as fixing the intra-site links, adding redirects etc.

We will lock up changes to zh/zh-cn pages for the moment, until this one gets in.

This PR is based on commit cdad0a7342.

content/zh-cn/docs/tasks/access-application-cluster/configure-dns-cluster.md

@@ -0,0 +1,32 @@
+---
+title: 为集群配置 DNS
+weight: 120
+content_type: concept
+---
+
+<!--
+---
+title: Configure DNS for a Cluster
+weight: 120
+content_type: concept
+---
+-->
+
+<!-- overview -->
+<!--
+Kubernetes offers a DNS cluster addon, which most of the supported environments enable by default. In Kubernetes version 1.11 and later, CoreDNS is recommended and is installed by default with kubeadm.
+-->
+Kubernetes 提供 DNS 集群插件，大多数支持的环境默认情况下都会启用。
+在 Kubernetes 1.11 及其以后版本中，推荐使用 CoreDNS，
+kubeadm 默认会安装 CoreDNS。
+
+<!-- body -->
+<!--
+For more information on how to configure CoreDNS for a Kubernetes cluster, see the [Customizing DNS Service](/docs/tasks/administer-cluster/dns-custom-nameservers/). An example demonstrating how to use Kubernetes DNS with kube-dns, see the [Kubernetes DNS sample plugin](https://github.com/kubernetes/examples/tree/master/staging/cluster-dns)
+-->
+要了解关于如何为 Kubernetes 集群配置 CoreDNS 的更多信息，参阅 
+[定制 DNS 服务](/zh/docs/tasks/administer-cluster/dns-custom-nameservers/)。
+关于如何利用 kube-dns 配置 kubernetes DNS 的演示例子，参阅 
+[Kubernetes DNS 插件示例](https://github.com/kubernetes/examples/tree/master/staging/cluster-dns)。
+
+