[COOK-4591] Implement chef_vault_secret resource

[COOK-4591](http://tickets.opscode.com/browse/COOK-4591)

This commit introduces the chef_vault_secret resource, with test-kitchen convergence support and post-convergence tests using ServerSpec.

In order to avoid awkward user creation that doesn't have a private key Chef Vault can use the public key for the secret, a special node/client is used (hydroelectric). The private key used here was generated so there is actually valid data such that Chef Vault works. This is a valid RSA private key, but it shouldn't be used anywhere outside testing this cookbook, as publishing it makes it immediately not secure for any purpose. You probably shouldn't use it for testing, but it's okay :-).

* add a suite to .kitchen.yml
* add the provider and resource as libraries subclassing LWRPBase
* create a test cookbook recipe to setup everything required to test that the resource will create an encrypted data bag item that uses ChefVault:
  - install build-essential (required for cheffish dependency)
  - install cheffish
  - create the data bag using cheffish (chef_data_bag)