TripleO (OSP < 10) allows you to specify 'NeutronTenantMtu' which sets path_mtu in neutron.conf, but that applies only for tunneled networks. We need the 'NeutronGlobalPhysnetMtu' which introduced in OSP 10, to allow setting the MTU for flat and vlan networks (global_physnet_mtu).
My patch that undid some of the tangled mess with regards to dhcp options would have been better off leaving the hieradata setting alone. It was either already directed to global_physnet_mtu at that point or was changed later
Is this relevant still for OSP 8?
Unless I am missing something, OSP 8 doesn't actually have global_physnet_mtu so ultimately we need a mechanism which produces similar results. FWICT that is segment_mtu? What's the actual impact/use of network_device_mtu in that release? I'm guessing it should be labelled "considered harmful".
Director sets up MTU correctly on OSP 10+, we are not able to fix this in older versions due to where the branches are in their lifecycle and our own team's Engineering capacity.