rkt: skip retrieving network information when net=none
rkt: skip retrieving network information when net=none

Even when net=none we would attempt to retrieve network information from
rkt which would spew useless log lines such as:

```
testlog.go:30: 20:37:31.409209 [DEBUG] driver.rkt: failed getting network info for pod UUID 8303cfe6-0c10-4288-84f5-cb79ad6dbf1c attempt 2: no networks found. Sleeping for 970ms
```

It would also delay tests for ~60s during the network information retry
period.

So skip this when net=none. It's unlikely anyone actually uses net=none
outside of tests, so I doubt anyone will notice this change.

Official docs:
https://coreos.com/rkt/docs/latest/networking/overview.html#no-loopback-only-networking

dvusboy/nomad,dvusboy/nomad,nak3/nomad,hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,dvusboy/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,nak3/nomad,nak3/nomad,hashicorp/nomad,burdandrei/nomad,nak3/nomad,nak3/nomad,hashicorp/nomad,hashicorp/nomad,dvusboy/nomad,dvusboy/nomad,nak3/nomad,nak3/nomad,burdandrei/nomad