Mark box_optional test as temporarily pending

test/unit/plugins/kernel_v2/config/vm_test.rb

@@ -100,6 +100,8 @@ describe VagrantPlugins::Kernel_V2::VMConfig do
     end
 
     it "is not required if the provider says so" do
+      # TODO: reenable this test once provider options are implemented
+      pending "removal of temporary workaround"
       machine.provider_options[:box_optional] = true
       subject.box = nil
       subject.finalize!