update openstack version to zed (kubernetes#2044)

Author: jichenjc

tests/playbooks/roles/install-devstack/defaults/main.yaml

@@ -1,7 +1,7 @@
 ---
 user: "stack"
 workdir: "/home/{{ user }}/devstack"
-branch: "stable/xena"
+branch: "stable/zed"
 enable_services:
   - nova
   - glance