Support network_device_mtu of nova managed interfaces

network_device_mtu configure the MTU for nova managed network interfaces.
By default no specific network_device_mtu configuration is set. The
interface defaults will be used as they got created.

This options allows configuration of Jumbo Frame usage.

Change-Id: Ia391f3daa788c780c4492071a480052b4ef4cc37
Merge "Support network_device_mtu of nova managed interfaces"