Add simple DSL method to interact with encrypted data bags.

Resolves https://github.com/opscode/chef/issues/1710.

\cc @opscode/client-engineers 
Once https://github.com/opscode/chef/blob/master/lib/chef/config.rb#L418-L429 is taken care of, we don't need the ` || Chef::Config.platform_specific_path('/etc/chef/encrypted_data_bag_secret')`