network v2 schema too permissive

With netplan uninstalled, given a config like:
```
network:
version: 2
ethernets:
eth0:
somekey: somevalue
```

Running `cloud-init schema --system` yields
```
Found cloud-config data types: user-data, network-config

1. user-data at /var/lib/cloud/instances/a1747431-4e15-4388-80b5-a32ccaeeaaa0/cloud-config.txt:
Valid schema user-data

2. network-config at /var/lib/cloud/instances/a1747431-4e15-4388-80b5-a32ccaeeaaa0/network-config.json:
Valid schema network-config
```

even though we should see a warning about `somekey` being invalid.