Preinstall used core providers plugins

[gberche-orange]

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