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/cluster.md

@@ -0,0 +1,50 @@
+---
+title: 集群（Cluster）
+id: cluster
+date: 2019-06-15
+full_link: 
+short_description: >
+   集群由一组被称作节点的机器组成。这些节点上运行 Kubernetes 所管理的容器化应用。集群具有至少一个工作节点。
+
+aka: 
+tags:
+- fundamental
+- operation
+---
+
+<!-- 
+---
+title: Cluster
+id: cluster
+date: 2019-06-15
+full_link: 
+short_description: >
+   A set of worker machines, called nodes, that run containerized applications. Every cluster has at least one worker node.
+
+aka: 
+tags:
+- fundamental
+- operation
+--- 
+-->
+
+<!-- 
+A set of worker machines, called {{< glossary_tooltip text="nodes" term_id="node" >}},
+that run containerized applications. Every cluster has at least one worker node.
+-->
+集群是由一组被称作{{< glossary_tooltip text="节点（node）" term_id="node" >}}的机器组成，
+这些节点上会运行由 Kubernetes 所管理的容器化应用。
+且每个集群至少有一个工作节点。
+<!--more-->
+<!-- 
+The worker node(s) host the {{< glossary_tooltip text="Pods" term_id="pod" >}} that are
+the components of the application workload. The
+{{< glossary_tooltip text="control plane" term_id="control-plane" >}} manages the worker
+nodes and the Pods in the cluster. In production environments, the control plane usually
+runs across multiple computers and a cluster usually runs multiple nodes, providing
+fault-tolerance and high availability.
+-->
+工作节点会托管所谓的 Pods，而 Pod 就是作为应用负载的组件。
+控制平面管理集群中的工作节点和 Pods。
+为集群提供故障转移和高可用性，
+这些控制平面一般跨多主机运行，而集群也会跨多个节点运行。