ml2override

::user_variables::

neutron_ml2_conf_ini_overrides:
  ml2:
    path_mtu:9000
    physical_network_mtus:'i40e:9000'

Note: Quoted to stop parser from complaining

::End Result::

[DEFAULT]
ml2 = path_mtu:9000 physical_network_mtus:'i40e:9000'

[ml2]
type_drivers = flat,vlan,vxlan,local
tenant_network_types = vxlan,vlan
mechanism_drivers = linuxbridge,l2population
extension_drivers = port_security

::Desired State::

[ml2]
type_drivers = flat,vlan,vxlan,local
tenant_network_types = vxlan,vlan
mechanism_drivers = linuxbridge,l2population
extension_drivers = port_security
path_mtu = 9000
physical_network_mtus = i40e:9000