Fix name of VirtualBox provider in Vagrantfile

This arose from a backport where in later branches we have changed the
name of the local variable used to refer to the VirtualBox
provider. This commit addresses this switching to the name we use in
future branches in case anything else is backported.

Vagrantfile

@@ -201,10 +201,10 @@ def provision(config,
   raise ArgumentError.new('update_command is required') if update_command == 'required'
   raise ArgumentError.new('update_tracking_file is required') if update_tracking_file == 'required'
   raise ArgumentError.new('install_command is required') if install_command == 'required'
-  config.vm.provider "virtualbox" do |v|
+  config.vm.provider "virtualbox" do |vbox|
     # Give the box more memory and cpu because our tests are beasts!
-    v.memory = Integer(ENV['VAGRANT_MEMORY'] || 8192)
-    v.cpus = Integer(ENV['VAGRANT_CPUS'] || 4)
+    vbox.memory = Integer(ENV['VAGRANT_MEMORY'] || 8192)
+    vbox.cpus = Integer(ENV['VAGRANT_CPUS'] || 4)
 
     # see https://github.com/hashicorp/vagrant/issues/9524
     vbox.customize ["modifyvm", :id, "--audio", "none"]