Setting path_mtu to reflect mtu for physical devices that handle traffic
issued from br-tun makes ml2 plugin to calculate mtu for tenant networks
properly, considering encapsulation headers. After that, calculated mtu
values can be propagated into instances (currently, only DHCP approach
is implemented; RA support for IPv6 subnets is under review).
This change allows to run tunnelled tenant networks in multinode when
underlying physical devices don't support jumbo frames.
Note: changing the default value in neutron would not be backwards
compatible, since it could slow down east-west tunnelled traffic in
clouds that run on jumbo-aware networks.
Change-Id: I8287677c7ad0f13fa9f5cb194f9372d04b78cb61
Related-Bug: #1527675
06cfce3756