Run infra doc build/publish jobs on Xenial

Move these jobs onto Xenial so that they have a newer home to live in.
These aren't associated with a true running service, instaed they build
files and possibly copy them elsewhere for hosting. Should be fine to
xenial as a result.

Change-Id: If9bd101513bd6a1300de846cc4bb4938af371736

jenkins/jobs/infra-publish-jobs.yaml

@@ -1,6 +1,6 @@
 - job-template:
     name: '{name}-infra-docs'
-    node: 'ubuntu-trusty'
+    node: 'ubuntu-xenial'
 
     builders:
         - print-template-name:
@@ -27,7 +27,7 @@
 - job-template:
     name: '{name}-infra-docs-tags-only'
     description: Publish infra documents, use when only publish on tag
-    node: ubuntu-trusty
+    node: ubuntu-xenial
 
     builders:
         - print-template-name:
@@ -55,7 +55,7 @@
 - job:
     name: publish-infra-docs-index
     description: Render and publish the infra docs index.
-    node: ubuntu-trusty
+    node: ubuntu-xenial
 
     builders:
       - gerrit-git-prep
@@ -80,7 +80,7 @@
 - job:
     name: gate-infra-docs-index
     description: Render the infra docs index without publishing it.
-    node: ubuntu-trusty
+    node: ubuntu-xenial
 
     builders:
       - gerrit-git-prep