Handle IP only access

We reference $DOKKU_ROOT/VHOST in domains before it was created in nginx-vhosts. So let's set it up straight away instead.

Also disables VHOSTs if HOSTNAME is an IP

closes #826
@michaelshobbs @josegonzalez I have a question here.

Why was `NO_VHOST` introduced? Previously, simply not creating a `dokku/VHOST` file would imply a IP based dokku.

The reason I ask is that it seems to be adding complexity we might be able to avoid.