Autodetect encrypted data bag items in data_bag_item dsl method.

OK, ready to go @opscode/client-engineers. This resolves https://github.com/opscode/chef/issues/1849.


---
Older messages
* :construction: Would like feedback on expected behavior @sersut @danielsdeleo @lamont-granquist :construction: 

Should we move these methods into encryptor classes like `Version1Encryptor` classes and query them from here to get the required keys? I think this would make be a little bit more clear and make sure that when we are writing `Version4Encryptor` we don't need to remember that we need to add a function here.