provisioners/chef: error if knife not found and auto-deleting

plugins/provisioners/chef/config/chef_client.rb

@@ -1,5 +1,7 @@
 require File.expand_path("../base", __FILE__)
 
+require "vagrant/util/which"
+
 module VagrantPlugins
   module Chef
     module Config
@@ -50,6 +52,12 @@ module VagrantPlugins
           errors << I18n.t("vagrant.config.chef.validation_key_path") if \
             !validation_key_path
 
+          if delete_client || delete_node
+            if !Vagrant::Util::Which.which("knife")
+              errors << I18n.t("vagrant.chef_config_knife_not_found")
+            end
+          end
+
           { "chef client provisioner" => errors }
         end
       end

templates/locales/en.yml

@@ -33,6 +33,9 @@ en:
       stdout: %{stdout}
 
       stderr: %{stderr}
+    chef_config_knife_not_found: |-
+      The `knife` application was not found! This is required by Vagrant
+      to automatically delete Chef nodes and clients.
     chef_run_list_empty: |-
       Warning: Chef solo run list is empty. This may not be what you want.
     provisioner_cleanup: |-