Use the infra mirrors for DIB

This patch configures DIB to use the infra mirrors when running
the gate jobs.

Depends-On: https://review.openstack.org/604933
Change-Id: I3293def6e6ce03db0627b425cbc77149d8e0170a
diff --git a/zuul.d/jobs.yaml b/zuul.d/jobs.yaml
index d1414b1..15147fa 100644
--- a/zuul.d/jobs.yaml
+++ b/zuul.d/jobs.yaml
@@ -57,6 +57,8 @@
       - openstack/diskimage-builder
       - openstack/python-barbicanclient
     vars:
+      devstack_localrc:
+        DIB_LOCAL_ELEMENTS: openstack-ci-mirrors
       devstack_services:
         barbican: true
         neutron-qos: true