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/reference/glossary/kubeadm.md

@@ -0,0 +1,42 @@
+---
+title: Kubeadm
+id: kubeadm
+date: 2018-04-12
+full_link: /zh/docs/setup/production-environment/tools/kubeadm/
+short_description: >
+  用来快速安装 Kubernetes 并搭建安全稳定的集群的工具。
+
+aka: 
+tags:
+- tool
+- operation
+---
+
+<!--
+title: Kubeadm
+id: kubeadm
+date: 2018-04-12
+full_link: /docs/admin/kubeadm/
+short_description: >
+  A tool for quickly installing Kubernetes and setting up a secure cluster.
+
+aka: 
+tags:
+- tool
+- operation
+-->
+
+<!--
+ A tool for quickly installing Kubernetes and setting up a secure cluster.
+-->
+用来快速安装 Kubernetes 并搭建安全稳定的集群的工具。
+
+<!--more--> 
+
+<!--
+You can use kubeadm to install both the control plane and the {{< glossary_tooltip text="worker node" term_id="node" >}} components.
+-->
+你可以使用 kubeadm 安装控制面和
+{{< glossary_tooltip text="工作节点" term_id="node" >}}
+组件。
+