fix(contrib/coreos): configure public IP for fleet

We previously defined this, but we did so by overwriting the
stock CoreOS unit file for fleet. That's bad. This enables us
to tweak this setting and still use the stock CoreOS unit file,
which is good.

This is necessary only for Vagrant, but it is safe for cloud
providers as well.

TESTING: provision multi-node, and make sure `fleetctl list-machines`
shows different IPs:
```console
$ fleetctl list-machines
MACHINE		IP		METADATA
4cbcb7c5...	172.17.8.101	-
e00803ec...	172.17.8.100	-
```
Works on EC2 as well...
```
fleetctl list-machines
MACHINE		IP		METADATA
2c985b00...	172.31.12.86	-
43cca2ad...	172.31.35.1	-
ecf3fc08...	172.31.31.144	-
```