optionally pull in a pre-built VM image as per the EVW packer branch

This change would allow starting with a clean directory (no VMs) but accelerate building the bootstrap node by pulling in a pre-built image that has been built with Packer. For safety, it still goes ahead and configures the NICs on the resulting VM isolating it from any possible differences in configurations the archived machine may have. Providing the packer build as an optional pre-build step is envisaged to be a later pull request.