[BENTO-97] Specify a Virtualbox VM name for each Packer template

The current packer templates do not contain `vm_name` and therefore Packer is defaulting the virtualbox name to be set as [packer-virtualbox](https://github.com/mitchellh/packer/blob/master/builder/virtualbox/builder.go#L153)

The use case for this is when you're trying to build multiple templates concurrently, it won't work:

    echo "centos-5.10-x86_64.json\ncentos-6.4-x86_64.json" | xargs -P20 -I% packer build -only=virtualbox %

Virtualbox will already see a VM named `packer-virtualbox` and will only build `centos-5.10-x86_64`.

I [already signed the CLA](https://wiki.opscode.com/display/chef/Approved+Contributors), number `1462`.

Thanks!