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/ubuntu-16.04/template.json b/ubuntu-16.04/template.json
index 84e1a59..284237e 100644
--- a/ubuntu-16.04/template.json
+++ b/ubuntu-16.04/template.json
@@ -4,6 +4,7 @@
     "password": "ho5uo7Uome5d",
     "distro": "ubuntu-16-04-x64",
     "disk_size": "8000",
+    "images_cache": "{{ env `PACKER_IMAGES_CACHE` }}",
     "ubuntu_baseurl": "https://mirror.mirantis.com/proposed/ubuntu/",
     "saltstack_gpg": "{{ env `SALTSTACK_GPG` }}",
     "saltstack_repo": "{{ env `SALTSTACK_REPO` }}",
@@ -284,6 +285,7 @@
       "iso_checksum": "566efef1d6f12e7d3a994c2405bdb642",
       "iso_checksum_type": "md5",
       "iso_url": "http://cloud-images.ubuntu.com/releases/xenial/release-20180306/ubuntu-16.04-server-cloudimg-amd64-disk1.img",
+      "iso_target_path": "{{ user `images_cache`}}/release-20180306_xenial-server-cloudimg-amd64-disk1.img",
       "disk_image": true,
       "disk_compression": true,
       "accelerator": "kvm",