Due many patterns of Linux distributions, it is difficult to install
ansible dependencies as system-wide stably.
Apart of Kubespray doc[1] recommends to use venv to avoid such issue,
and this applies venv usage to the other parts of the doc.

[1]: https://github.com/kubernetes-sigs/kubespray/blob/master/docs/setting-up-your-first-cluster.md#set-up-kubespray