publishers: added option to define groovy postbuild script

Change-Id: I3ee3be40148321412c04bfa2c72152bc41986389
Reviewed-on: https://review.openstack.org/20905
Reviewed-by: Jaroslav Henner <jhenner@redhat.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins