Property file for parameterized downstream jobs

Added ability to specify parameters from properties file for parameterized
build on other projects.  This change follows the predefined-properties code
as a template for how to do the work.  This is a change to only the
publishers.py file.

Change-Id: I4b3500dc9b4f50450475aa8441854fa6b5b0ccb0
Reviewed-on: https://review.openstack.org/23432
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins