Comment 3 for bug 1826368

Also note the workaround mentioned in:
https://ask.openstack.org/en/question/121359/error-neutron-gateway-in-openstack-on-lxd/?answer=121385#post-id-121385

-- setting sysctl to the empty string explicitly.

juju config neutron-gateway sysctl= juju resolve neutron-gateway/0