Run logstash-filters job on Xenial

This job was running on Xenial because we set it up using puppet which
we don't support on bionic. The default nodeset was then switched to
bionic. Change this job back to xenial.

Change-Id: Ide5614c550486e3d34f9217c3f03781aebd58e75

zuul.d/zuul-legacy-jobs.yaml

@@ -506,6 +506,7 @@
     post-run: playbooks/legacy/logstash-filters/post.yaml
     timeout: 1800
     voting: false
+    nodeset: ubuntu-xenial
 
 - job:
     name: legacy-logstash-filters-ubuntu-trusty