builder/virtualbox-ovf: fix shutdown_timeout handling

The shutdown_timeout config parameter was being ignored
(for both the user setting and the "5m" default) on the
virtualbox-ovf builder. As a result, packer complained
of timeouts on graceful shutdowns, and the build process
would error out. This fixes that along with some newly
created tests.

website: correct default puppet provisioner command

Documentation/docker: link pointing to dockerfiles was incorrect

website: added missing builders to intro

builders/digitalocean: private networking