Fix puppet unit ci config

We no longer use puppet 4.x so lets drop that job. Additionally we made
the 6.0 version voting but forgot to add it to the gate.

Change-Id: I1e891b8d7b4be7fc679cc352473079354a917c4b

zuul.d/layout.yaml

@@ -2,7 +2,6 @@
     name: puppet-tripleo-module-unit-jobs
     check:
       jobs:
-        - puppet-openstack-unit-4.8-centos-7
         - puppet-openstack-unit-5.5-centos-7
         - puppet-openstack-unit-6.0-ubuntu-bionic:
             voting: false
@@ -10,8 +9,8 @@
             voting: false
     gate:
       jobs:
-        - puppet-openstack-unit-4.8-centos-7
         - puppet-openstack-unit-5.5-centos-7
+        - puppet-openstack-unit-6.0-ubuntu-bionic
 
 - project:
     templates: