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/daemonset.md

@@ -0,0 +1,43 @@
+---
+title: DaemonSet
+id: daemonset
+date: 2018-04-12
+full_link: /zh/docs/concepts/workloads/controllers/daemonset/
+short_description: >
+  确保 Pod 的副本在集群中的一组节点上运行。
+
+aka: 
+tags:
+- fundamental
+- core-object
+- workload
+---
+
+<!--
+---
+title: DaemonSet
+id: daemonset
+date: 2018-04-12
+full_link: /docs/concepts/workloads/controllers/daemonset
+short_description: >
+  Ensures a copy of a Pod is running across a set of nodes in a cluster.
+
+aka: 
+tags:
+- fundamental
+- core-object
+- workload
+---
+-->
+<!--
+ Ensures a copy of a {{< glossary_tooltip text="Pod" term_id="pod" >}} is running across a set of nodes in a {{< glossary_tooltip text="cluster" term_id="cluster" >}}.
+-->
+ 确保 {{< glossary_tooltip text="Pod" term_id="pod" >}} 的副本在{{< glossary_tooltip text="集群" term_id="cluster" >}}中的一组节点上运行。
+<!--more--> 
+
+<!--
+Used to deploy system daemons such as log collectors and monitoring agents that typically must run on every {{< glossary_tooltip term_id="node" >}}.
+-->
+
+用来部署系统守护进程，例如日志搜集和监控代理，这些进程通常必须运行在每个{{< glossary_tooltip text="节点" term_id="node" >}}上。
+