[setup] Write systemd network config

[dmach]

When running kanku setup, it would be great to write a config file for people who use split dns via systemd-resolved.
That would make the kanku domain and DNS server persistent.

My config looks like this:
$ cat /etc/systemd/network/virbr1.conf

[Match]
Name=virbr1

[Resolve]
DNS=192.168.36.1
Domains=kanku.site

The file could be named kanku.conf in case subsequent kanku setup runs change the interface name.