Fix imcomplete migration to CentOS8 in ut/lint

The previous change[1] was merged to migrate jobs from CentOS7 to
CentOS8, but it was in fact incomplete regarding the following points.
 - Check jobs run on CentOS8, while gate job only run on CentOS7
 - Library jobs are still voted by CentOS7 jobs

This patch fixes the imcomplete update to make sure that we use
CentOS8 for master jobs.

[1] fc189a89739c71f8cb4b8b3ba7c60333ff0735c6

Depends-on: https://review.opendev.org/#/c/715336/
Change-Id: If47b8ec2a43fbf53079ed9029f339a8f904e763e

zuul.d/libraries.yaml

@@ -12,6 +12,7 @@
 - job:
     name: puppet-openstack-libraries-puppet-unit-5.5-centos-7
     parent: puppet-openstack-libraries-puppet-unit-base
+    branches: ^stable/(ocata|pike|queens|rocky|stein|train)$
     nodeset: centos-7
     vars:
       puppet_gem_version: 5.5.0
@@ -20,7 +21,6 @@
     name: puppet-openstack-libraries-puppet-unit-5.5-centos-8
     parent: puppet-openstack-libraries-puppet-unit-base
     nodeset: centos-8
-    voting: false
     branches: ^(?!stable/(ocata|pike|queens|rocky|stein)).*$
     vars:
       puppet_gem_version: 5.5.0
@@ -29,7 +29,6 @@
     name: puppet-openstack-libraries-puppet-unit-6.11-centos-8
     parent: puppet-openstack-libraries-puppet-unit-base
     nodeset: centos-8
-    voting: false
     branches: ^(?!stable/(ocata|pike|queens|rocky|stein)).*$
     vars:
       puppet_gem_version: 6.11.0

zuul.d/unit.yaml

@@ -67,4 +67,6 @@
       jobs:
         - puppet-openstack-unit-4.10-centos-7
         - puppet-openstack-unit-5.5-centos-7
+        - puppet-openstack-unit-5.5-centos-8
+        - puppet-openstack-unit-6.11-centos-8
         - puppet-openstack-unit-6.11-ubuntu-bionic