Keep federation jobs running on Xenial

Devstack/Tempest are going to migrate to the latest Ubuntu LTS release
18.04 aka Bionic, but there is an issue with the apache shibboleth
module, see [1] and [2].

As a workaround we keep the federation jobs running on Xenial for now.

[0] https://etherpad.openstack.org/p/devstack-bionic
[1] https://bugs.launchpad.net/keystone/+bug/1802901
[2] https://bugs.launchpad.net/ubuntu/+source/xmltooling/+bug/1776489

Change-Id: I8ad93577b1e5a5084e2a65cb1377eb7b0d353cb7
Depends-On: https://review.openstack.org/620906

.zuul.yaml

@@ -35,6 +35,7 @@
 - job:
     name: keystone-dsvm-functional-federation
     parent: keystone-dsvm-functional
+    nodeset: openstack-single-node-xenial
     vars:
       devstack_localrc:
         TEMPEST_PLUGINS: '/opt/stack/keystone-tempest-plugin'