Update spec/item_keys_spec.rb

* Change classname to ChefVault::VaultItemKeys
* Add clear_clients test
Updates to item_spec.rb

* Changed ChefVault::Item to ChefVault::VaultItem
* Added require 'chefspec' for additional test methods (tested against
  chefspec-3.4.0/rspec-2.14)
* Changes for key refreshing:
  * Added tests for new :refresh action in clients method
  * Added tests for new add_server method

For the above two tests, I added somewhat extensive stubbing and mocking
to make the tests as comprehensive as possible within the rspec/chefspec
execution environment.
Add tests for new refresh method add_admin_user to item_spec.rb
Add test for key refreshing method refresh! in item_spec.rb
Add methods to refresh keys based on saved search_query

In vault_item.rb:
* Added :refresh action to clients method.  Clears the existing client
  keys (for security and to prevent runaway increase in keys) then
  re-runs the saved search_query to re-populate
* refresh! method: Wrapper method to  re-run clients with :refresh action.
* add_server method: Support method for knife to add the chef-server's
  client key to the key list in order to allow chef recipes running on
  the server to refresh the key list.  Takes a search query and checks
  that the matching nodes have the correct attribute value at the
  top-level.  (Because of the way chef's SOLR search is configured, all
  attributes are flattened.  There is no way to specify, for example,
  that 'tags:server' only matches node[:tags] and not
  node[:some][:nested][:attribute][:tags].  By forcing a search on
  top-level attributes only, which cannot be set by cookbooks recipes,
  the integrity of the list and the security of the vault items is
  maintained.)
* add_admin_user method: Support method for knife to add the 'admin'
  user's key to facilitate key updates by multiple users.

In vault_item_keys.rb:
* clear_clients method: removes all existing client keys
Update knife modules to allow client key refresh

* Added knife_refresh.rb to refresh the clients keys on a vault item
* Added --add-server-key option to vault_create.rb and vault_update.rb.
  It takes a SOLR search query intended to match the chef-server,
  allowing the chef-server's client key to be added to the vault item so
  the keys can be refreshed automatically via a recipe