Cannot override nameservers, only append to list

[Jouni K. Seppänen]

The netplan reference states:

> Lexicographically later files (regardless of in which directory they are)
> amend (new mapping keys) or override (same mapping keys) previous ones.

On a freshly installed Ubuntu 18.04 server on DigitalOcean, cloud-init creates a configuration file `/etc/netplan/50-cloud-init.yaml`, which includes something like this:

```
network:
    ethernets:
        eth0:
            nameservers:
                addresses:
                - 8.8.8.8
                - 8.8.4.4
                search: []
```

I created my own configuration file, with a lexicographically later name and the same mapping keys with different values:

```
network:
    ethernets:
        eth0:
            nameservers:
                addresses:
                - 9.9.9.9
```

The result was that `systemd-resolve --status` listed the union of the nameserver lists for `eth0` instead of only the latter ones. Thus while the mapping key was the same, the values didn't override the previous ones but amended them.

Some discussion at https://askubuntu.com/questions/1070278/how-can-i-override-netplans-per-interface-nameservers-rather-than-appending-to