[COOK-4560] Attribute to disable ChefVault

https://tickets.opscode.com/browse/COOK-4560

As odd as this option might sound, I think it's worth moving towards one solution that allows use for either chef-vault, or built-in Chef::EncryptedDataBagItem objects.

Example would be chef-splunk cookbook.  It uses chef-vault, so those not using ChefVault would have to implement wrappers just to disable the vault usage.  Whereas this option would make both parties happy.