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/node.md

@@ -0,0 +1,50 @@
+---
+title: 节点（Node）
+id: node
+date: 2018-04-12
+full_link: /zh/docs/concepts/architecture/nodes/
+short_description: >
+  Kubernetes 中的工作机器称作节点。
+
+aka: 
+tags:
+- fundamental
+---
+
+<!--
+title: Node
+id: node
+date: 2018-04-12
+full_link: /zh/docs/concepts/architecture/nodes/
+short_description: >
+  A node is a worker machine in Kubernetes.
+
+aka: 
+tags:
+- fundamental
+-->
+
+<!--
+A node is a worker machine in Kubernetes.
+-->
+Kubernetes 中的工作机器称作节点。
+
+<!--more--> 
+
+<!--
+A worker node may be a VM or physical machine, depending on the cluster. It has local daemons or services necessary to run {{< glossary_tooltip text="Pods" term_id="pod" >}} and is managed by the control plane. The daemons on a node include {{< glossary_tooltip text="kubelet" term_id="kubelet" >}}, {{< glossary_tooltip text="kube-proxy" term_id="kube-proxy" >}}, and a container runtime implementing the {{< glossary_tooltip text="CRI" term_id="cri" >}} such as {{< glossary_tooltip term_id="docker" >}}.
+
+In early Kubernetes versions, Nodes were called "Minions".
+-->
+工作机器可以是虚拟机也可以是物理机，取决于集群的配置。
+其上部署了运行 {{< glossary_tooltip text="Pods" term_id="pod" >}}
+所必需的本地守护进程或服务，
+并由主控组件来管理。
+节点上的的守护进程包括 {{< glossary_tooltip text="kubelet" term_id="kubelet" >}}、
+{{< glossary_tooltip text="kube-proxy" term_id="kube-proxy" >}}
+以及一个 {{< glossary_tooltip term_id="docker" >}} 这种
+实现了 {{< glossary_tooltip text="CRI" term_id="cri" >}}
+的容器运行时。
+
+在早期的 Kubernetes 版本中，节点也称作 "Minions"。
+