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/ingress.md

@@ -0,0 +1,46 @@
+---
+title: Ingress
+id: ingress
+date: 2018-04-12
+full_link: /zh/docs/concepts/services-networking/ingress/
+short_description: >
+  Ingress 是对集群中服务的外部访问进行管理的 API 对象，典型的访问方式是 HTTP。
+
+aka: 
+tags:
+- networking
+- architecture
+- extension
+---
+
+<!--
+---
+title: Ingress
+id: ingress
+date: 2018-04-12
+full_link: /docs/concepts/services-networking/ingress/
+short_description: >
+  An API object that manages external access to the services in a cluster, typically HTTP.
+
+aka: 
+tags:
+- networking
+- architecture
+- extension
+---
+-->
+
+<!--
+ An API object that manages external access to the services in a cluster, typically HTTP.
+-->
+
+  Ingress 是对集群中服务的外部访问进行管理的 API 对象，典型的访问方式是 HTTP。
+
+<!--more--> 
+
+<!--
+Ingress may provide load balancing, SSL termination and name-based virtual hosting.
+-->
+
+Ingress 可以提供负载均衡、SSL 终结和基于名称的虚拟托管。
+