Add openstack-single-node-xenial nodeset

Allow other jobs to explicitly require a node running Xenial. This seems
clearer than having a generic openstack-single-node nodeset which
implicitly uses Xenial.

Change-Id: I013fb8abd4e6ab6539bd9410acbc8446e57ec70c

.zuul.yaml

@@ -18,6 +18,16 @@
         nodes:
           - controller
 
+- nodeset:
+    name: openstack-single-node-xenial
+    nodes:
+      - name: controller
+        label: ubuntu-xenial
+    groups:
+      - name: tempest
+        nodes:
+          - controller
+
 - nodeset:
     name: devstack-single-node-centos-7
     nodes: