Support for virtualbox and windows 2012
Update README.md
VMware Updates, Reformatting JSON

- Updating GuestOS value for Windows Server 2018 to windows8srv-64
- Setting headless to true for all provisioners
- Updating VMware Tools to latest version
VirtualBox builders install VBoxGuestAdditions
Merge pull request #2 from dylanmei/vbox_guest_additions

VirtualBox builders install VBoxGuestAdditions