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/security-context.md

@@ -0,0 +1,52 @@
+---
+title: 安全上下文（Security Context）
+id: security-context
+date: 2018-04-12
+full_link: /zh/docs/tasks/configure-pod-container/security-context/
+short_description: >
+  securityContext 字段定义 Pod 或容器的特权和访问控制设置，包括运行时 UID 和 GID。
+
+aka: 
+tags:
+- security
+---
+
+<!--
+---
+title: Security Context
+id: security-context
+date: 2018-04-12
+full_link: /docs/tasks/configure-pod-container/security-context/
+short_description: >
+  The securityContext field defines privilege and access control settings for a Pod or container.
+
+aka: 
+tags:
+- security
+---
+-->
+
+<!--
+ The `securityContext` field defines privilege and access control settings for
+a {{< glossary_tooltip text="Pod" term_id="pod" >}} or
+{{< glossary_tooltip text="container" term_id="container" >}}.
+-->
+
+securityContext 字段定义 {{< glossary_tooltip text="Pod" term_id="pod" >}} 或
+{{< glossary_tooltip text="容器" term_id="container" >}}的特权和访问控制设置。
+
+<!--more-->
+
+<!--
+In a `securityContext`, you can define: the user that processes run as,
+the group that processes run as, and privilege settings.
+You can also configure security policies (for example: SELinux, AppArmor or seccomp).
+-->
+
+在一个 `securityContext` 字段中，你可以设置进程所属用户和用户组、权限相关设置。你也可以设置安全策略（例如：SELinux、AppArmor、seccomp）。	
+
+<!--
+The `PodSpec.securityContext` setting applies to all containers in a Pod.
+-->
+
+`PodSpec.securityContext` 字段配置会应用到一个 Pod 中的所有的 container 。