The only way, as far as I know, to run a task on all servers, is to omit the `roles` parameter. Setting it to `nil`, `[]` or `[:all]` won't work. Do you know a value that can be passed that will tell Capistrano to use all roles?

Otherwise, I could set the default to `[:app, :web, :db]` but if someone set a custom role, it won't be included.