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