Merge branch 'master' of https://github.com/kubernetes/kubernetes.github.io into release-1.7

* 'master' of https://github.com/kubernetes/kubernetes.github.io:
  Document how to use custom certs with kubeadm (#4113)
  Document product_uuid and MAC reqs

docs/setup/independent/install-kubeadm.md

@@ -13,6 +13,7 @@ This page shows how to use install kubeadm.
 * One or more machines running Ubuntu 16.04+, CentOS 7 or HypriotOS v1.0.1+
 * 1GB or more of RAM per machine (any less will leave little room for your apps)
 * Full network connectivity between all machines in the cluster (public or private network is fine)
+* Unique MAC address and product_uuid for every node
 
 {% endcapture %}
 
@@ -26,7 +27,7 @@ Versions 1.13 and 17.03+ have not yet been tested and verified by the Kubernetes
 For installation instructions, see
 [Install Docker](https://docs.docker.com/engine/installation/).
 
-## Installing kubectl 
+## Installing kubectl
 
 On each of your machines,
 [install kubectl](/docs/tasks/tools/install-kubectl/).