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.
Merge pull request #407 from bcrochet/bz/1151438

BZ #1151438: Add support for network_device_mtu and veth_mtu