CreateNetworkStatuses should assign IP addresses to a default interface if one isn't specified.

[Brian-McM]

Multus 4.1 started using CreateNetworkStatuses instead of CreateNetworkStatus in this PR. This broke the current default behaviour where an IP is assigned to a default address if it doesn't specify an interface index.

Upgrading multus to 4.1 breaks when using calico (and possibly other CNIs) since it doesn't specify the interface index. Calico should probably be updated to specific the interface index just to be explicit, but I think it is reasonable to have a default, especially since:

1. This used to default the IPs to an interface
2. It would be reasonable to default to an eth0 interface, just like containerd does here

In conjunction with this, I think it would also be reasonable to fail (and probably should fail) if it can't assign the IP addresses to an interface instead of silently dropping the IPs.