FR - provide terraform or vagrant executor for using VMs

Similar to #318 (closed), it would be really nice to have the runner support something like Terraform or Vagrant for spinning up VMs.

Reasoning

• not everyone/company are ready/wanting to use Docker.
• not everything works well or at-all in Docker (systemd for example)

Using something like Terraform or Vagrant, you would get the same benefits as docker-machine, i.e. a tool that has multiple drivers (In the case of Vagrant you'd need the relevant plugin - e.g. https://github.com/mitchellh/vagrant-aws, https://github.com/devopsgroup-io/vagrant-digitalocean) supporting different platforms. You can choose your own image, mount volumes etc. Logically this could replace the existing Virtualbox executor, which is a bit unwieldy tbh as it requires an existing image to clone and is locked to that.

I envisage something that

• generates a vagrant file, defining
  • driver
  • image
  • custom shell provisioning (script section of .gitlab-ci.yml)
  • optional volume mounts
• runs vagrant up
• runs vagrant destroy