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/etcd.md

@@ -0,0 +1,48 @@
+---
+title: etcd
+id: etcd
+date: 2018-04-12
+full_link: /zh/docs/tasks/administer-cluster/configure-upgrade-etcd/
+short_description: >
+  etcd 是兼具一致性和高可用性的键值数据库，用作保存 Kubernetes 所有集群数据的后台数据库。
+
+aka: 
+tags:
+- architecture
+- storage
+---
+
+<!--
+---
+title: etcd
+id: etcd
+date: 2018-04-12
+full_link: /docs/tasks/administer-cluster/configure-upgrade-etcd/
+short_description: >
+  Consistent and highly-available key value store used as Kubernetes' backing store for all cluster data.
+
+aka: 
+tags:
+- architecture
+- storage
+---
+-->
+
+<!--
+ Consistent and highly-available key value store used as Kubernetes' backing store for all cluster data.
+-->
+
+`etcd` 是兼顾一致性与高可用性的键值数据库，可以作为保存 Kubernetes 所有集群数据的后台数据库。
+
+<!--more--> 
+<!--
+If your Kubernetes cluster uses etcd as its backing store, make sure you have a
+[back up](/docs/tasks/administer-cluster/configure-upgrade-etcd/#backing-up-an-etcd-cluster) plan
+for those data.
+-->	
+你的 Kubernetes 集群的 `etcd` 数据库通常需要有个[备份](/zh/docs/tasks/administer-cluster/configure-upgrade-etcd/#backing-up-an-etcd-cluster)计划。
+<!--
+You can find in-depth information about etcd in the official [documentation](https://etcd.io/docs/).
+-->
+
+如果想要更深入的了解 `etcd`，请参考 [etcd 文档](https://etcd.io/docs/)。