Specify that the cluster.yml playbook should run as root (#3474)

* Specify that the cluster.yml playbook should run as root

This is a possible fix for #3388.
The following examples show the option `-b` too:

https://kubernetes.io/docs/setup/custom-cloud/kubespray/
https://github.com/kubernetes-incubator/kubespray/blob/master/docs/getting-started.md#starting-custom-deployment

* Update invocation to include specific root user

* Update comment text according to suggestions

README.md

@@ -33,8 +33,11 @@ To deploy the cluster you can use :
     cat inventory/mycluster/group_vars/all/all.yml
     cat inventory/mycluster/group_vars/k8s-cluster/k8s-cluster.yml
 
-    # Deploy Kubespray with Ansible Playbook
-    ansible-playbook -i inventory/mycluster/hosts.ini cluster.yml
+    # Deploy Kubespray with Ansible Playbook - run the playbook as root
+    # The option `-b` is required, as for example writing SSL keys in /etc/,
+    # installing packages and interacting with various systemd daemons.
+    # Without -b the playbook will fail to run!
+    ansible-playbook -i inventory/mycluster/hosts.ini --become --become-user=root cluster.yml
 
 Note: When Ansible is already installed via system packages on the control machine, other python packages installed via `sudo pip install -r requirements.txt` will go to a different directory tree (e.g. `/usr/local/lib/python2.7/dist-packages` on Ubuntu) from Ansible's (e.g. `/usr/lib/python2.7/dist-packages/ansible` still on Ubuntu).
 As a consequence, `ansible-playbook` command will fail with: