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/rbac.md

@@ -0,0 +1,43 @@
+---
+title: 基于角色的访问控制（RBAC）
+id: rbac
+date: 2018-04-12
+full_link: /zh/docs/reference/access-authn-authz/rbac/
+short_description: >
+  管理授权决策，允许管理员通过 Kubernetes API 动态配置访问策略。
+
+aka: 
+tags:
+- security
+- fundamental
+---
+
+<!--
+---
+title: RBAC (Role-Based Access Control)
+id: rbac
+date: 2018-04-12
+full_link: /zh/docs/reference/access-authn-authz/rbac/
+short_description: >
+  Manages authorization decisions, allowing admins to dynamically configure access policies through the Kubernetes API.
+
+aka: 
+tags:
+- security
+- fundamental
+---
+-->
+
+<!--
+ Manages authorization decisions, allowing admins to dynamically configure access policies through the {{< glossary_tooltip text="Kubernetes API" term_id="kubernetes-api" >}}.
+-->
+管理授权决策，允许管理员通过 {{< glossary_tooltip text="Kubernetes API" term_id="kubernetes-api" >}} 动态配置访问策略。
+
+<!--more--> 
+
+<!--
+RBAC utilizes *roles*, which contain permission rules, and *role bindings*, which grant the permissions defined in a role to a set of users.
+-->
+RBAC 使用 *角色* (包含权限规则)和 *角色绑定* (将角色中定义的权限授予一组用户)。
+
+