Ensure kubeadm doesn't use proxy (#7275)

* Move proxy_env to kubespray-defaults/defaults

There is no reasons to use set_facts here

Signed-off-by: Etienne Champetier <e.champetier@ateme.com>

* Ensure kubeadm doesn't use proxy

*_proxy variables might be present in the environment (/etc/environment, bash profile, ...)
When this is the case we end up with those proxy configuration in /etc/kubernetes/manifests/kube-*.yaml manifests

We cannot unset env variables, but kubeadm is nice enough to ignore empty vars
https://github.com/kubernetes/kubernetes/blob/93d288e2a47fa6d497b50d37c8b3a04e91da4228/cmd/kubeadm/app/util/env.go#L27



Signed-off-by: Etienne Champetier <e.champetier@ateme.com>