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

https://tickets.opscode.com/browse/CHEF-5356
> Since this is a new version of this feature, how about we require a minimum secret length?

In my first implementation, I added a warning if the secret was to short. But I removed in the final PR because I really did not know what to choose as recommended length.

In the [Official Data Bags documentation](http://docs.opscode.com/chef/essentials_data_bags.html#secret-keys), a secret length of **684 bytes** is used. Maybe double that size?