util/packer: Refactor scripts and use single JSON template

This is so `packer` can build both Vagrant and EC2 images in parallel.
Yep we still [run the upgrade script first](https://github.com/flynn/flynn/blob/release-system/util/packer/ubuntu-14.04/template.json#L70) which [does the reboot](https://github.com/flynn/flynn/blob/release-system/util/packer/ubuntu-14.04/scripts/upgrade.sh#L10).