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/container-env-variables.md

@@ -0,0 +1,40 @@
+---
+title: 容器环境变量（Container Environment Variables）
+id: container-env-variables
+date: 2018-04-12
+full_link: /zh/docs/concepts/containers/container-environment/
+short_description: >
+  容器环境变量提供了 name=value 形式的、运行容器化应用所必须的一些重要信息。
+
+aka: 
+tags:
+- fundamental
+---
+
+<!--
+---
+title: Container Environment Variables
+id: container-env-variables
+date: 2018-04-12
+full_link: /docs/concepts/containers/container-environment/
+short_description: >
+  Container environment variables are name=value pairs that provide useful information into containers running in a Pod.
+
+aka: 
+tags:
+- fundamental
+---
+-->
+
+<!--
+ Container environment variables are name=value pairs that provide useful information into containers running in a {{< glossary_tooltip text="pod" term_id="pod" >}}
+-->
+
+  容器环境变量提供了 name=value 形式的、在 {{< glossary_tooltip text="pod" term_id="pod" >}} 中运行的容器所必须的一些重要信息。
+  
+<!--more-->
+<!--
+Container environment variables provide information that is required by the running containerized applications along with information about important resources to the {{< glossary_tooltip text="containers" term_id="container" >}}. For example, file system details, information about the container itself, and other cluster resources such as service endpoints.
+-->
+
+  容器环境变量为运行中的容器化应用提供必要的信息，同时还提供与 {{< glossary_tooltip text="容器" term_id="container" >}} 重要资源相关的其他信息，例如：文件系统信息、容器自身的信息以及其他像服务端点（Service endpoints）这样的集群资源信息。