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/control-plane.md

@@ -0,0 +1,53 @@
+---
+title: 控制平面（Control Plane）
+id: control-plane
+date: 2019-05-12
+full_link:
+short_description: >
+  控制平面是指容器编排层，它暴露 API 和接口来定义、部署容器和管理容器的生命周期。
+
+aka:
+tags:
+- fundamental
+---
+<!--
+title: Control Plane
+id: control-plane
+date: 2019-05-12
+full_link:
+short_description: >
+  The container orchestration layer that exposes the API and interfaces to define, deploy, and manage the lifecycle of containers.
+
+aka:
+tags:
+- fundamental
+-->
+
+<!--
+ The container orchestration layer that exposes the API and interfaces to define, deploy, and manage the lifecycle of containers.
+-->
+控制平面（Control Plane）是指容器编排层，它暴露 API 和接口来定义、
+部署容器和管理容器的生命周期。
+
+<!--more-->
+
+<!--
+ This layer is composed by many different components, such as (but not restricted to):
+
+ * {{< glossary_tooltip text="etcd" term_id="etcd" >}}
+ * {{< glossary_tooltip text="API Server" term_id="kube-apiserver" >}}
+ * {{< glossary_tooltip text="Scheduler" term_id="kube-scheduler" >}}
+ * {{< glossary_tooltip text="Controller Manager" term_id="kube-controller-manager" >}}
+ * {{< glossary_tooltip text="Cloud Controller Manager" term_id="cloud-controller-manager" >}}
+
+ These components can be run as traditional operating system services (daemons) or as containers. The hosts running these components were historically called {{< glossary_tooltip text="masters" term_id="master" >}}.
+-->
+这个编排层是由多个不同的组件组成，例如以下（但不限于）几种：
+
+ * {{< glossary_tooltip text="etcd" term_id="etcd" >}}
+ * {{< glossary_tooltip text="API 服务器" term_id="kube-apiserver" >}}
+ * {{< glossary_tooltip text="调度器" term_id="kube-scheduler" >}}
+ * {{< glossary_tooltip text="控制器管理器" term_id="kube-controller-manager" >}}
+ * {{< glossary_tooltip text="云控制器管理器" term_id="cloud-controller-manager" >}}
+
+这些组件可以以传统的系统服务运行也可以以容器的形式运行.运行这些组件的主机过去称为 master 节点。