Comment 2 for bug 1836313

bridge_mappings is set as public:br-ex initially:
https://github.com/openstack/stx-config/blob/master/kubernetes/applications/stx-openstack/stx-openstack-helm/stx-openstack-helm/manifests/manifest.yaml#L1201

If no normal datanetwork is configured, the bridge_mappings won't be changed and ovs agent will fail to start.