Add preprovisioned base box for Drupal VM

[thom8]

Unfortunately, Hashicorp are decommissioning their packer cloud service which this project has been using to build base images.

The Packer, Artifact Registry and Terraform Enterprise (Legacy) features of Atlas will 
no longer be actively developed or maintained and will be fully decommissioned 
on Thursday, May 31, 2018

Rather than migrating to another service it would be nice if we could add a preprovisioned base box for Drupal VM and we wouldn't need to maintain seperate projects.

geerlingguy/drupal-vm#1774