Release k8s v1.12.8

Task: 30675
Story: 2005124

Change-Id: I97ca231d70308862ed8b84d9364a6028c43e3382

playbooks/container-builder-vars.yaml

@@ -1,8 +1,8 @@
-magnum_src_dir: "src/git.openstack.org/openstack/magnum"
+magnum_src_dir: "src/opendev.org/openstack/magnum"
 
 magnum_repository: openstackmagnum
 kubernetes_version_v1_11: v1.11.9
-kubernetes_version_v1_12: v1.12.7
+kubernetes_version_v1_12: v1.12.8
 kubernetes_version_v1_13: v1.13.5
 kubernetes_version_v1_14: v1.14.1
 kubernetes_version_v1_15: v1.15.0-alpha.1