Use `Chef::Config` to get the default secret file path.

This cookbook no longer works starting with chef 11.6, I believe, since the constant `Chef::EncryptedDataBagItem::DEFAULT_SECRET_FILE` has been removed in
favor of `Chef::Config[:encrypted_data_bag_secret]` (commit is opscode/chef@64b1a49, and discussion is at https://tickets.opscode.com/browse/CHEF-4011).

This change updates the attribute to use the `Chef::Config` value instead.