make ip optional for configured hosts

This PR makes the IP address optional for static hosts. If the IP address is omitted the machine IP is used.

The use case for this is being able to configure multiple hosts withoint managing static IPs e.g. 

```ruby
config.vm.network :public_network
    
config.landrush.enable
config.landrush.host 'foo.example.com'
config.landrush.host 'bar.example.net'
config.landrush.host 'baz.example.org'
```