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. After the `gsub`, you need

    domain_name '_'
    domain_name << env[:machine].name.to_s

to add the individual machine's name.