Since a790935d all proxy users
should be properly configured

Now when you have *_PROXY vars in your environment it can leads to failure
if NO_PROXY is not correct, or to persistent configuration changes
as seen with kubeadm in 1c5391dd

Instead of playing constant whack-a-bug, inject empty *_PROXY vars everywhere
at the play level, and override at the task level when needed

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