Refactor packer ci templates

Improve maintainability by refactoring some common code.

- Improve documentation
- Reorganize anchors for better maintainability
- Remove dependency on jjb_common anchor
  (updates to jjb code should not not break packer code)
- Do not run packer merge jobs concurrently
  (merge jobs should not be configured to run concurrently)

Change-Id: Ib796cfee44ae7b01b42ee400a04be79b243cac3b
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>