Ensure MTU is set when the OVS vif driver is used

Set the MTU on the relevant devices when the OVS vif driver
is used:
    - veth devices
    - ovs vif device

Change-Id: I217f43227c099f17ab9a225b1ef7377d3de28114
Closes-bug: 1260697
Merge "Ensure MTU is set when the OVS vif driver is used"