script: Update release-vm-images to release EC2 AMIs

This builds EC2 AMIs using packer and releases a manifest similar to the vagrant one, e.g.:

```json
{
  "name": "flynn-base",
  "versions": [
    {
      "version": "20141021.2",
      "images": [
        {
          "hypervisor": "xen",
          "id": "ami-aa6bc5dd",
          "name": "flynn-20141021.2-ubuntu-14.04-1413920729",
          "region": "eu-west-1",
          "owner_id": "061747782411",
          "root_device_type": "ebs",
          "root_device_name": "/dev/sda1",
          "root_device_snapshot_id": "snap-f3c0550e",
          "virtualization_type": "hvm"
        },
        {
          "hypervisor": "xen",
          "id": "ami-765eda1e",
          "name": "flynn-20141021.2-ubuntu-14.04-1413920729",
          "region": "us-east-1",
          "owner_id": "061747782411",
          "root_device_type": "ebs",
          "root_device_name": "/dev/sda1",
          "root_device_snapshot_id": "snap-1893f6a1",
          "virtualization_type": "hvm"
        }
      ]
    }
  ]
}
```