juju br0/juju-br0 does not observe dhcp mtu settings

In our MAAS based openstack deployment, we provide MTU via DHCP to all servers to enable jumbo frames.

This works fine until juju creates the bridge devices for LXC or KVM containers, at which point we reliably lose the MTU 9000 setting on the bridge, resulting in all sorts of MTU related fun in things like corosync and percona-cluster which are running under lxc.

We've fixed this by appending an explicit mtu set to the bridge definition in /etc/network/interfaces:

auto juju-br0
iface juju-br0 inet dhcp
    bridge_ports eth0
    post-up /sbin/ip link set eth0 mtu 9000

After making this change, we can reliably reboot physical hosts container lxc containers, an not break our clusters.

The issue is that MTU has to be set on the underlying device, not the bridge itself - this may actually be something we can fix in the dhcpclient tools