Neutron agents do not use network MTU

Backport for version 7.0:
https://review.fuel-infra.org/18069

IMO, this patch adds a feature to product, but besides that there are large concerns about its applicability to stable versions of MOS. There are a bunch of MTU-related fixes in community's master branch. I have spoken with Eugene Nikanorov and we agreed that we shouldn't port only one patch and all MTU-related patches are non-backportable. Hence, I will close this issue as Won't Fix (wontfix-risky).I ask those who will consider to re-open this bug, please check the links below first:
[1] https://specs.openstack.org/openstack/fuel-specs/specs/7.0/jumbo-frames-between-instances.html
[2] https://review.openstack.org/#/q/owner:%22Kevin+Benton%22+status:merged
[3] https://review.openstack.org/#/c/284814/ -- Add global_physnet_mtu and deprecate segment_mtu
[4] https://review.openstack.org/#/c/283798/ -- Deprecate network_device_mtu
[5] https://review.openstack.org/#/c/283847/ -- Use MTU value from Neutron in OVS/LB VIF wiring
[6] https://review.openstack.org/#/c/286449/ -- Set veth_mtu default to 9000