Fix empty encrypted_data_bag_secret_key_path

If there's no `encrypted_data_bag_secret_key_path` set in the
`Vagrantfile`, `@config.encrypted_data_bag_secret_key_path` returns an
`Object`, and Vagrant tries to upload the key nonetheless.

This commit checks that `@config.encrypted_data_bag_secret_key_path` is
a `String` before attempting the upload.