Use ZUUL_ prefix instead of GERRIT_ prefix in jobs

* modules/jenkins/files/slave_scripts/create-ppa-package.sh:
* modules/jenkins/files/slave_scripts/docbook-properties.sh: Use
ZUUL_REFNAME instead of GERRIT_REFNAME as the jenkins gearman plugin
does not receive GERRIT_* parameters.

Change-Id: I5869b277e9344b8805997a3c702e1ae3af0a9e8f
Reviewed-on: https://review.openstack.org/34052
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins