Add random hostname option

Let the user add information to the hostname from their Vagrantfile.

When on, prevent domain name collisions.

Off by default.

Fixes #215
As this point domain_name is just the prefix, which is shared between all VMs in the Vagrantfile. Before appending the random string, you need

```
domain_name << '_'
domain_name << env[:machine].name.to_s
```

to add the individual machine's name.