[WiP] Cloudstack driver

It's a Work-In-Progress PR, help is welcome !

While we also use https://github.com/mindjiver/gopherstack for having a CloudStack client in go (used in https://github.com/mindjiver/packer-cloudstack), i assume that we can use that lib.

I'm a big beginner in Go, so don't hesitate to point me what could be wrong :)