Add chef-vault support for the application data bag

This adds an extra attribute: chef_vault, which if set to true, uses
chef-vault (https://github.com/Nordstrom/chef-vault) rather than data_bag_item
in order to load the supermarket data, allowing it to be encrypted on the chef
server.

I've done basic testing with test kitchen with and without the attribute enabled. With the chef_vault attribute disabled, everything works as it did before. With the chef_vault attribute enabled, it fails trying to load from the (non-existent) vault, but otherwise works fine.