Add tripleo container build job

The tripleo continainer build process consumes the kolla project. We
would like to add a non-voting job for kolla as the start of better
integration between the two.  This job runs the tripleo build process
which calls kolla-build under the covers.  Failures in this job might
indicate breakages in the kolla build api.

Change-Id: I5ce7c5888858f535eee0b5539c1e2d7978a7438e

.zuul.d/centos.yaml

@@ -12,6 +12,11 @@
             required-projects:
               - openstack/kolla-ansible
               - openstack/requirements
+        - tripleo-build-containers-centos-7:
+            voting: false
+            files:
+              - ^docker/.*$
+              - ^kolla/.*$
     gate:
       jobs:
         - kolla-build-centos-source