Support kvm-libvirt terraform module

[pothos]

The kvm-libvirt platform is not yet supported in lokoctl.
Expose it in lokoctl and create docs which cover what was written in https://github.com/kinvolk/lokomotive-kubernetes/blob/master/docs/flatcar-linux/kvm-libvirt.md

[pothos]

Can this be part of https://github.com/kinvolk/lokomotive/milestone/4?

[invidian]

Would be great if we could implement it after #392 is merged, so we can test new, reusable Terraform modules which #392 is introducing. This should simplify adapting kvm-libvirt code and avoid extending duplication of Terraform code.