Comment 2 for bug 1533942

br-ex is created by this task ansible/roles/neutron/tasks/start.yml:

...
- name: Ensuring OVS bridge is properly setup
  command: docker exec openvswitch_db /usr/local/bin/kolla_ensure_openvswitch_configured {{ neutron_bridge_name }} {{ neutron_external_interface }}
  register: status
  changed_when: status.stdout.find('changed') != -1
  when:
    - (inventory_hostname in groups['compute'] or inventory_hostname in groups['neutron-agents'])
    - neutron_plugin_agent == "openvswitch"
...

And in the kolla_ensure_openvswitch_configured, br-ex will be created:
    ovs-vsctl --no-wait add-br $bridge

This run after openvswitch_db but before openvswitch_vswitchd.

I think the ovs-vsctl should run after openvswitch_vswitchd and proposed a fix.

That fixed the problem but Sam points out there should be no dependency, so root cause is still unknown.