Add the gerrit-git-prep macro so gitdm code actually gets cloned first

Change-Id: I5d441475d13471a38540d2051a81f404e4f27c0d
Reviewed-on: https://review.openstack.org/31722
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins

modules/openstack_project/files/jenkins_job_builder/config/gitdm.yaml

@@ -6,6 +6,7 @@
       - zuul
 
     builders:
+      - gerrit-git-prep
       - shell: |
           #!/bin/bash -xe
           export TEMPDIR=${TEMPDIR:-$(mktemp -d $(pwd)/dmtmp-XXXXXX)}
@@ -26,6 +27,7 @@
       - timed: '0 6 * * *'
 
     builders:
+      - gerrit-git-prep
       - shell: |
           #!/bin/bash -xe
           export TEMPDIR=${TEMPDIR:-$(mktemp -d $(pwd)/dmtmp-XXXXXX)}