Partial backport: Disable broken image building

These periodic jobs are failing, let's disable the periodic jobs.
They have been broken since ages without fixing. There's no
record of a successful run under Zuul v3.
Last images at http://tarballs.openstack.org/magnum/images/ are from
2017.

This only backports the disabling of the jobs.

Change-Id: I01122fa029b4124d912e80ea43bca07b8f2ebe5c
(cherry picked from commit 42c75c4d3d)

.zuul.yaml

@@ -296,8 +296,10 @@
         - magnum-functional-swarm-ironic
         - magnum-functional-k8s-multinode
         - magnum-functional-swarm-mode-multinode
-    periodic:
-      jobs:
-        - magnum-dib-buildimage-fedora-atomic-25
-        - magnum-dib-buildimage-ubuntu-mesos
-        - magnum-dib-buildimage-centos-dcos
+    # XXX: The periodic jobs are completely broken and need to be fixed,
+    # enable them once they work again.
+    # periodic:
+    #   jobs:
+    #     - magnum-dib-buildimage-fedora-atomic-25
+    #     - magnum-dib-buildimage-ubuntu-mesos
+    #     - magnum-dib-buildimage-centos-dcos