update to centos-8

tripleo ussuri / master is
centos-8 only now. centos-7
jobs will be removed.

Change-Id: I46eb7b168019f62baeec20d5eab72a463ddb9526

zuul.d/project.yaml

@@ -29,8 +29,3 @@
     gate:
       jobs:
         - tobiko-devstack-functional
-
-    experimental:
-      jobs:
-        - tobiko-devstack-faults-centos-7-queens
-        - tobiko-devstack-faults-centos-7-train

zuul.d/tobiko-tripleo.yaml

@@ -2,7 +2,7 @@
     name: tobiko-tripleo
     abstract: true
     description: Base Tobiko TripleO job
-    parent: tripleo-ci-centos-7-standalone
+    parent: tripleo-ci-centos-8-scenario002-standalone
     required-projects:
       - openstack/tripleo-ci
       - x/tobiko