Add support for the promoted builds plugin.

Change-Id: I393f1c4d47253fbe53f774c5e60cd20f65b5348e
Reviewed-on: https://review.openstack.org/28005
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
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