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
12 files changed