Improve packer-validate debugging output

This patch achieves 3 tasks:

1. Improves the debugging output provided by printing out the
   details about which vars and templates files are being tested.
2. Fixes an issue where packer was not actually testing every
   combination of varfile and template file due to cloud-env.json.
3. Includes common-packer vars files if available (should silently
   ignore if it does not exist).

Issue: RELENG-744
Change-Id: Id6e1f2423a378cbe376ccefe2568c22669fb7859
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>