BZ #1151438: Add support for network_device_mtu and veth_mtu

https://bugzilla.redhat.com/show_bug.cgi?id=1151438

Expose the network_device_mtu param on neutron and the veth_mtu param on
neutron::agents::ovs in order to properly set the mtu in the appropriate
configurations. Both params default to undef, so if not specified, there
will be no change in behaviour.