Add proper qemu-caching for source image

  * Require command at slaves:
    'mkdir -p "${HOME}/packer_images_cache/"'
    since packer not able to do it.
  * set SALT_STOPSTART_WAIT=10 for salt bootstrap
  * Dd iptables to base installation

Change-Id: Icc5a50a21e848b93660aa80483b6fdd3e27de243
diff --git a/mirror-image/template.json b/mirror-image/template.json
index 5abc542..48045c1 100644
--- a/mirror-image/template.json
+++ b/mirror-image/template.json
@@ -3,6 +3,7 @@
     "user": "root",
     "password": "r00tme",
     "disk_size": "100000",
+    "images_cache": "{{ env `PACKER_IMAGES_CACHE` }}",
     "do_api_token": "{{ env `DO_API_TOKEN` }}",
     "image_name": "{{ env `IMAGE_NAME` }}",
     "cluster_model": "{{ env `CLUSTER_MODEL` }}",
@@ -151,7 +152,7 @@
       "accelerator": "kvm",
       "disk_size": "{{ user `disk_size`}}",
       "iso_url": "http://cloud-images.ubuntu.com/releases/xenial/release-20180306/ubuntu-16.04-server-cloudimg-amd64-disk1.img",
-      "iso_target_path": "/root/isos/xenial-server-cloudimg-amd64-disk1.img",
+      "iso_target_path": "{{ user `images_cache`}}/release-20180306_xenial-server-cloudimg-amd64-disk1.img",
       "iso_checksum_type": "md5",
       "disk_image": true,
       "disk_compression": true,