Fixes that `containers.<name>.extraVeths.<name>` configuration was not
always applied.

When configuring `containers.<name>.extraVeths.<name>` and not
configuring one of `containers.<name>.localAddress`, `.localAddress6`,
`.hostAddress`, `.hostAddress6` or `.hostBridge` the veth was created,
but otherwise no configuration (i.e. no ip) was applied.

nixos-container always configures the primary veth (when `.localAddress`
or `.hostAddress` is set) to be the containers default gateway, so
this fix is required to create a veth in containers that use a different
default gateway.

To test this patch configure the following container and check if the
addresses are applied:
```
  containers.testveth = {
    extraVeths.testveth = {
      hostAddress = "192.168.13.2";
      localAddress = "192.168.13.1";
    };
    config = {...}:{};
  };
```

Fix: nixos-container does not always apply extraVeth ips