Support for vmware_workstation and customizing vmware memory/CPUs

Hey there, these are some quick changes to:

1. add support for the `vmware_workstation` provider
2. enable customizing memory/CPUs in vmware fusion/workstation using the existing `$vb_memory` and `$vb_cpus` config options
3. update README instructions accordingly (and a minor correction for setup step numbering)
Is there any reason you are using strings here instead of symbols?