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
diff --git a/.zuul.yaml b/.zuul.yaml
index 7ac30d8..f3fc87f 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -19,6 +19,16 @@
           - 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:
       - name: controller