[CHEF-5356] Encrypted data bags should use different HMAC key and include the IV in the HMAC

https://tickets.opscode.com/browse/CHEF-5356
Because the used _SHA256_ HMAC has a 64 bytes block length and _SHA512_ outputs a 64 digest length. Using the a key with the length of the block [seems to be the recommended approach](http://en.wikipedia.org/wiki/Hash-based_message_authentication_code#Implementation).