Make integration jobs non-voting and version-pinned

The upstream base job has been switched from Xenial to Bionic, causing
our integration job to fail. Make it non-voting while we work on fixing
it and also introduce a job that runs on Xenial to ensure we keep
testing there.

Change-Id: Ie7981ee522f3b73bb62a2ce02ed247c2b7a33458

.zuul.yaml

@@ -8,25 +8,34 @@
     timeout: 2700
 
 - job:
-    name: openstack-chef-integration
+    name: openstack-chef-integration-bionic
     parent: base
-    description: Run integration tests with openstack-chef
+    nodeset: ubuntu-bionic
+    description: Run integration tests with openstack-chef on Ubuntu Bionic
     required-projects: openstack/openstack-chef
     pre-run: playbooks/pre.yaml
     run: playbooks/integration.yaml
     post-run: playbooks/post.yaml
     timeout: 3600
 
+- job:
+    name: openstack-chef-integration-xenial
+    parent: openstack-chef-integration-bionic
+    nodeset: ubuntu-xenial
+    description: Run integration tests with openstack-chef on Ubuntu Xenial
+
 - project-template:
     name: openstack-chef-jobs
     check:
       jobs:
         - openstack-chef-delivery
-        - openstack-chef-integration
+        - openstack-chef-integration-bionic:
+            voting: false
+        - openstack-chef-integration-xenial:
+            voting: false
     gate:
       jobs:
         - openstack-chef-delivery
-        - openstack-chef-integration
 
 - project:
     templates: