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

Closes-bug: 1260697

Conflicts:

	nova/network/linux_net.py

Change-Id: I217f43227c099f17ab9a225b1ef7377d3de28114
(cherry picked from commit e34a085f7a524747dd93e9645c6e0e69631075d4)
Merge "Ensure MTU is set when the OVS vif driver is used" into stable/havana