Update the packer templates in releng/builder

Update the packer templates in releng/builder

- Add the arch type to image name, which is required for
  auto-updating images.
- Re-factor templates files with user vars similar to
  the templates in common-packer.

Change-Id: I78a63cddc857aabfa1e1177060267e4146d55ff0
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>