Preinstall used core providers plugins #2
Description
With terraform 0.10, core plugins are not distributed anymore with the terraform core distribution, and a terraform init downloads them from the internet.
We should presinstall modules such as openstack, aws, powerdns, cloudflare into the images as to avoid repetitive downloads.
See https://www.terraform.io/guides/running-terraform-in-automation.html#pre-installed-plugins