If the MTU of the netowrk of interface is 0,
"Netlink Error, invalid argument" will be raised,
and kury-cni exits unexpectedly.
When mtu is 0,
1. dhcp-option-force of dnsmasq for is not set,
and mtu of interface of vm is set to be 1500 by default
2. mtu of qvo-xxx and qvb-xxx is set to 1500,
which is determined by configuration,network_device_mtu.
Fix this by adding default network device MTU configuration,
which is used instead if MTU of network is 0
Change-Id: I7c1140b116aa6304aca13479a7d7eb82d828d16f
Close-Bug: 1738812
c00513bb64