Move NullArtifact to the packer base classes to reuse it
Add support for NullArtifact to qemu
Add support for NullArtifact to Amazon
Add support for NullArtifact to DigitalOcean
Add support for NullArtifact to Docker
Add support for NullArtifact to GoogleCompute
Add support for NullArtifact to OpenStack
Add support for NullArtifact to Parallels
Add support for NullArtifact to VirtualBox
Add support for NullArtifact to VMWare