Add simple DSL method to interact with encrypted data bags.

Resolves https://github.com/opscode/chef/issues/1710.

\cc @opscode/client-engineers 
I think it would be a good idea to extract a method for `default_encrypted_data_bag_secret_path` or something so we don't need to repeat `Chef::Config.platform_specific_path('/etc/chef/encrypted_data_bag_secret')` Also I think it needs to be interpolated rather than the literal code.