Add support for job priority sorter plugin.

This plugin provides a way to configure priority of jobs in
build queue. Each job has a inherent, configurable, priority.
Builds of higher priority are scheduled first.

Change-Id: Ie7f374369a141bec094bf9a6a7438082bf298fb5
Reviewed-on: https://review.openstack.org/35064
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins