Advertising correct MTU via IPv6 RA in OSP 7

Issue

MTU in IPv6 is advertised via router advertisements (RA). However, any try to advertise correct MTU to instances in OSP 7 / kilo, in
IPv6 only (not dual stack) does not seem to work.

None of the below steps (see "Diagnostic Steps") seems to advertise the correct MTU - neither on new networks, nor on networks which were created before the below changes. In fact, tcpdump shows that neutron routers don't advertise MTU at all.

• MTU is advertised via IPv6 RA, and not via DHCPv6

• This is the documentation for MTU settings in the overcloud: https://access.redhat.com/documentation/en/red-hat-enterprise-linux-openstack-platform/7/networking-guide/chapter-17-configure-mtu-settings