I revert a last patch I did about veth_mtu param.
veth_mtu enforces mtu on veths (between br-int and physnets only).
It's not useful in our case, where this is the external network which is
tagged, so we only need to enforce the MTU within the VM, using DHCP
options.
Close bug #270
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>