Add support for generating workshop lab host as a VMware virtual machine

[hodgesrm]

Hi!

Your docker lab is excellent. Some of us work with VMware, hence prefer to use a VMware virtual machine image rather than Amazon for the lab exercises. I committed changes containing packer files to generate VMware images and will submit a PR in a few minutes for your consideration. I verified that the script changes do not affect generation of AMIs, which still work as before.

I hope that once this PR is accepted it will make rubber-docker more accessible to the VMware community including folks at VMware itself.

Thanks again for the great work.

Robert