ci: run jjb-validate batch job against a PR

The batch job templates for OpenShift use a GIT_REF variable that is
used by the jobs to checkout the repository. This defaults to
"ci/centos", but it never is adjusted when jjb-validate runs against a
PR.

With the new GIT_REF environment variable in the jjb.sh script, the
variable can now be used to checkout a PR from gitHub and run the
validation against that.

Signed-off-by: Niels de Vos <ndevos@redhat.com>

deploy/jjb-validate.yaml

@@ -27,9 +27,13 @@ objects:
                 - name: GIT_REPO
                   value: https://github.com/ceph/ceph-csi
                 - name: GIT_REF
-                  value: ci/centos
+                  value: "${GIT_REF}"
           restartPolicy: Never
 parameters:
   - name: SESSION
     description: unique ID for the session to track the pod for the job
     required: true
+  - name: GIT_REF
+    description: the git branch or other ref to checkout and validate
+    value: ci/centos
+    required: false