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/volume-plugin.md

@@ -0,0 +1,46 @@
+---
+title: 卷插件（Volume Plugin）
+id: volumeplugin
+date: 2018-04-12
+full_link: 
+short_description: >
+  卷插件可以让 Pod 集成存储。
+
+aka: 
+tags:
+- core-object
+- storage
+---
+
+<!--
+---
+title: Volume Plugin
+id: volumeplugin
+date: 2018-04-12
+full_link: 
+short_description: >
+  A Volume Plugin enables integration of storage within a Pod.
+
+aka: 
+tags:
+- core-object
+- storage
+---
+-->
+
+<!--
+ A Volume Plugin enables integration of storage within a {{< glossary_tooltip text="Pod" term_id="pod" >}}.
+-->
+
+卷插件可以让 {{< glossary_tooltip text="Pod" term_id="pod" >}} 集成存储。
+
+<!--more--> 
+
+<!--
+A Volume Plugin lets you attach and mount storage volumes for use by a {{< glossary_tooltip text="Pod" term_id="pod" >}}. Volume plugins can be _in tree_ or _out of tree_. _In tree_ plugins are part of the Kubernetes code repository and follow its release cycle. _Out of tree_ plugins are developed independently.
+-->
+
+卷插件让你能给 {{< glossary_tooltip text="Pod" term_id="pod" >}} 附加和挂载存储卷。
+卷插件既可以是 _in tree_ 也可以是 _out of tree_ 。_in tree_ 插件是 Kubernetes 代码库的一部分，
+并遵循其发布周期。而 _Out of tree_ 插件则是独立开发的。
+