Add ability to restrict jobs to specific files.

Add an additional job parameter, 'file', that will cause that
job to only run if the change touches files that match the
specification.

Change-Id: I8c8fd3d029e02e338fd1dd266443b9ac56c0e5ac
Reviewed-on: https://review.openstack.org/23710
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins