Add ZUUL_COMMIT.

To facilitate using zuul with just the jenkins git plugin, add
the ZUUL_COMMIT parameter.  Also, always include ZUUL_REF, whether
the job is pre- or post-commit.  If it's pre, it will be a
refs/zuul ref, if it's post, it will be refs/tags or a branch
name.

Change-Id: I88c38a28dcd552b2540095d36caacd10acf167b8
Reviewed-on: https://review.openstack.org/13934
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins
6 files changed