Keep stable branch jobs on Ubuntu Xenial

Jobs for stable branches need to keep running on
Xenial, even though master switched to Bionic with [0] and [1].

[0] https://review.openstack.org/610977
[1] https://review.openstack.org/618169

Change-Id: I2e8bd9de75ea97ec05c5aba328b6033031bfa5f0
diff --git a/.zuul.yaml b/.zuul.yaml
index bd8619f..5c3477a 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -96,6 +96,7 @@
 
 - job:
     name: neutron-tempest-plugin-api-queens
+    nodeset: openstack-single-node-xenial
     parent: neutron-tempest-plugin-api
     override-checkout: stable/queens
     vars:
@@ -108,6 +109,7 @@
 
 - job:
     name: neutron-tempest-plugin-api-rocky
+    nodeset: openstack-single-node-xenial
     parent: neutron-tempest-plugin-api
     override-checkout: stable/rocky
     vars:
@@ -167,6 +169,7 @@
 - job:
     name: neutron-tempest-plugin-scenario-linuxbridge-queens
     parent: neutron-tempest-plugin-scenario-linuxbridge
+    nodeset: openstack-single-node-xenial
     override-checkout: stable/queens
     vars:
       branch_override: stable/queens
@@ -176,6 +179,7 @@
 - job:
     name: neutron-tempest-plugin-scenario-linuxbridge-rocky
     parent: neutron-tempest-plugin-scenario-linuxbridge
+    nodeset: openstack-single-node-xenial
     override-checkout: stable/rocky
     vars:
       branch_override: stable/rocky
@@ -310,6 +314,7 @@
 - job:
     name: neutron-tempest-plugin-dvr-multinode-scenario-queens
     parent: neutron-tempest-plugin-dvr-multinode-scenario
+    nodeset: openstack-two-node-xenial
     override-checkout: stable/queens
     vars:
       branch_override: stable/queens
@@ -317,6 +322,7 @@
 - job:
     name: neutron-tempest-plugin-dvr-multinode-scenario-rocky
     parent: neutron-tempest-plugin-dvr-multinode-scenario
+    nodeset: openstack-two-node-xenial
     override-checkout: stable/rocky
     vars:
       branch_override: stable/rocky
@@ -350,6 +356,7 @@
 - job:
     name: neutron-tempest-plugin-designate-scenario-queens
     parent: neutron-tempest-plugin-designate-scenario
+    nodeset: openstack-single-node-xenial
     override-checkout: stable/queens
     vars:
       branch_override: stable/queens
@@ -357,6 +364,7 @@
 - job:
     name: neutron-tempest-plugin-designate-scenario-rocky
     parent: neutron-tempest-plugin-designate-scenario
+    nodeset: openstack-single-node-xenial
     override-checkout: stable/rocky
     vars:
       branch_override: stable/rocky