images: use ubuntu minimal cloud image instead of regular

This significantly reduces image size, and since we're not using the
images interactively does not sacrifice anything we care about.

images/base.pkr.hcl

@@ -25,8 +25,8 @@ source "qemu" "autogenerated_1" {
   disk_image       = true
   headless         = true
   http_directory   = "scripts"
-  iso_checksum     = "file:https://cloud-images.ubuntu.com/bionic/current/SHA256SUMS"
-  iso_url          = "https://cloud-images.ubuntu.com/bionic/current/bionic-server-cloudimg-amd64.img"
+  iso_checksum     = "file:https://cloud-images.ubuntu.com/minimal/releases/bionic/release/SHA256SUMS"
+  iso_url          = "https://cloud-images.ubuntu.com/minimal/releases/bionic/release/ubuntu-18.04-minimal-cloudimg-amd64.img"
   memory           = "${var.memory}"
   net_device       = "virtio-net"
   output_directory = "output-${var.outname}"