Add a Bionic Beaver nodeset

I'm switching tempest/dsvm jobs to run on
Ubuntu 18.04 LTS (Bionic Beaver) on openstack/manila,
and I believe this nodeset can be here
so other projects can use it too.

Change-Id: Ib8279cde3e14d5378f27254188ee14dbb0800428
Needed-By: https://review.openstack.org/#/c/604929/

.zuul.yaml

@@ -8,6 +8,16 @@
         nodes:
           - controller
 
+- nodeset:
+    name: openstack-single-node-bionic
+    nodes:
+      - name: controller
+        label: ubuntu-bionic
+    groups:
+      - name: tempest
+        nodes:
+          - controller
+
 - nodeset:
     name: devstack-single-node-centos-7
     nodes: