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/dynamic-volume-provisioning.md

@@ -0,0 +1,46 @@
+---
+title: 动态卷供应（Dynamic Volume Provisioning）
+id: dynamicvolumeprovisioning
+date: 2018-04-12
+full_link: /zh/docs/concepts/storage/dynamic-provisioning/
+short_description: >
+  允许用户请求自动创建存储卷。
+
+aka: 
+tags:
+- core-object
+- storage
+---
+
+<!--
+---
+title: Dynamic Volume Provisioning
+id: dynamicvolumeprovisioning
+date: 2018-04-12
+full_link: /docs/concepts/storage/dynamic-provisioning
+short_description: >
+  Allows users to request automatic creation of storage  Volumes.
+
+aka: 
+tags:
+- core-object
+- storage
+---
+-->
+
+<!--
+ Allows users to request automatic creation of storage  {{< glossary_tooltip text="Volumes" term_id="volume" >}}.
+-->
+
+ 允许用户请求自动创建存储 {{< glossary_tooltip text="卷" term_id="volume" >}}。
+
+<!--more--> 
+
+<!--
+Dynamic provisioning eliminates the need for cluster administrators to pre-provision storage. Instead, it automatically provisions storage by user request. Dynamic volume provisioning is based on an API object, {{< glossary_tooltip text="StorageClass" term_id="storage-class" >}}, referring to a {{< glossary_tooltip text="Volume Plugin" term_id="volume-plugin" >}} that provisions a {{< glossary_tooltip text="Volume" term_id="volume" >}} and the set of parameters to pass to the Volume Plugin.
+-->
+
+动态供应让集群管理员无需再预先供应存储。相反，它通过用户请求自动地供应存储。
+动态卷供应是基于 API 对象 {{< glossary_tooltip text="StorageClass" term_id="storage-class" >}} 的，
+StorageClass 可以引用 {{< glossary_tooltip text="卷插件" term_id="volume-plugin" >}} 提供的
+{{< glossary_tooltip text="卷" term_id="volume" >}}，也可以引用传递给卷插件（Volume Plugin）的参数集。