CentOS 8: Enable seed upgrade job

Kolla CentOS 8 Train Bifrost image is now available.

Change-Id: If2b06f80fa2817377ed881aa61ef73e4be38df55
Story: 2006574
Task: 39047

zuul.d/project.yaml

@@ -13,8 +13,7 @@
         - kayobe-overcloud-centos8
         - kayobe-overcloud-upgrade-centos8
         - kayobe-seed-centos8
-        # TODO(mgoddard): Enable upgrade jobs when CentOS 8 support in Train.
-        # - kayobe-seed-upgrade-centos8
+        - kayobe-seed-upgrade-centos8
 
     gate:
       queue: kayobe
@@ -25,5 +24,4 @@
         - kayobe-overcloud-centos8
         - kayobe-overcloud-upgrade-centos8
         - kayobe-seed-centos8
-        # TODO(mgoddard): Enable upgrade jobs when CentOS 8 support in Train.
-        # - kayobe-seed-upgrade-centos8
+        - kayobe-seed-upgrade-centos8