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/cluster-infrastructure.md

@@ -0,0 +1,29 @@
+---
+title: 集群基础设施（Cluster Infrastructure）
+id: cluster-infrastructure
+date: 2019-05-12
+full_link:
+short_description: >
+ 基础设施层提供并维护虚拟机、网络、安全组及其他资源。
+
+aka:
+tags:
+- operations
+---
+
+<!--
+title: Cluster Infrastructure
+id: cluster-infrastructure
+date: 2019-05-12
+full_link:
+short_description: >
+ The infrastructure layer provides and maintains VMs, networking, security groups and others.
+
+aka:
+tags:
+- operation
+-->
+<!--
+The infrastructure layer provides and maintains VMs, networking, security groups and others.
+-->
+基础设施层提供并维护虚拟机、网络、安全组及其他资源。