have existing virtualbox virtual machines where I need to keep the mac
addresses so that the network interfaces come up when packer boots them.
Otherwise I just get SSH timeout and I'm unable to configure the machines.
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.