Comment 1 for bug 1825012

This can be done using config overrides in user_variable.yml, for example:

neutron_neutron_conf_overrides:
  DEFAULT:
    global_physnet_mtu: 9000
neutron_ml2_conf_ini_overrides:
  ml2:
    path_mtu: 1500

Since you need to set container_mtu in openstack-ansible manually, this looks like the best way to configure Neutron.