Fix host_manager plugin for vagrant virtualbox

There is no such thing as virtualbox_{ubuntu,centos} but only
a virtualbox provider. The code to retrieve the VM's IP address was
never executed and broke host_manager plugin.

TrivialFix

Change-Id: I0ffacf0bd5d2856264247b23e3b3e918836988e3

dev/vagrant/Vagrantfile

@@ -149,8 +149,7 @@ Vagrant.configure(2) do |config|
       if vm.name
         `python newest_dhcp_lease.py #{vm.name}`.chop
       end
-    when "virtualbox_ubuntu"
-    when "virtualbox_centos"
+    when "virtualbox"
       if vm.id
         `VBoxManage guestproperty get #{vm.id} "/VirtualBox/GuestInfo/Net/1/V4/IP"`.split()[1]
       end