Vagrant DNS proxy for VirtualBox

Use VirtualBox's builtin DNS proxy server. This ensures that the guest
doesn't see the real nameservers from the host's network, and will continue
to work when the host moves between physical locations. Cribbed from GOV.UK

Vagrantfile

@@ -7,6 +7,14 @@ Vagrant.configure("2") do |config|
   config.vm.box     = "precise64"
   config.vm.box_url = "http://files.vagrantup.com/precise64.box"
 
+  config.vm.provider :virtualbox do |vb|
+    modifyvm_args = ['modifyvm', :id]
+    # Isolate guests from host networking.
+    modifyvm_args << "--natdnsproxy1" << "on"
+    modifyvm_args << "--natdnshostresolver1" << "on"
+    vb.customize(modifyvm_args)
+  end
+
   config.vm.provision :puppet do |puppet|
     puppet.manifest_file  = "site.pp"
     puppet.manifests_path = "manifests"