Run legacy puppet 3 syntax checks on xenial

The base nodest has moved ahead to bionic but puppet 3 doesn't work on
bionic. Instead pin these legacy jobs to the legacy ubuntu xenial
nodeset so that we run these jobs on a platform that understands puppet
3.

Change-Id: Idecb985ef1e3ee89e65be3bf7873022e83fd1a6d

zuul.d/zuul-legacy-jobs.yaml

@@ -1024,6 +1024,7 @@
     parent: legacy-puppet-openstack-integration
     run: playbooks/legacy/puppet-syntax-3/run.yaml
     timeout: 1800
+    nodeset: legacy-ubuntu-xenial
 
 - job:
     name: legacy-puppet-syntax-3-centos-7
@@ -1268,6 +1269,7 @@
     parent: legacy-base
     run: playbooks/legacy/system-config-puppet-syntax-3/run.yaml
     timeout: 1800
+    nodeset: legacy-ubuntu-xenial
 
 - job:
     name: legacy-tempest-almanach-dsvm