Make report message introduction configurable.

By setting parameters on a pipeline, you can change the default
"Build successful/failed" introductory messages left on Gerrit.

Change-Id: Ie05c8c87a47994faced4ee67fda4ab4e64ea0d20
Reviewed-on: https://review.openstack.org/19677
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins