Comment 1 for bug 1821135

The tunnel interface is hard coded as docker0. You can find the code by following command:
   On active controller:
      export OS_CLOUD=openstack_helm
      kubectl -n openstack edit cm neutron-bin

The code is listed below:
    tunnel_interface="docker0"
    if [ -z "${tunnel_interface}" ] ; then
        # search for interface with default routing
        # If there is not default gateway, exit
        tunnel_interface=$(ip -4 route list 0/0 | awk -F 'dev' '{ print $2; exit }' | awk '{ print $1 }') || exit 1
    fi