[CHEF-3615] Make encrypted data bag format configurable

* Add an option to configure the version used when encrypting data bag
  items. This allows users to opt-in to newer encrypted data bag formats
  while the default remains compatible with earlier chef versions.
* Add an option to set a minimum valid encrypted data bag item format.
  This is useful on the client so that, for example, a MITM attacker
  cannot downgrade a v2 EDBI to v1.