provisioners/ansible: refer to inventory directory

By referring the directory that contains the generated inventory file,
users can easily provide more settings with additional files stored in
the same directory.

plugins/provisioners/ansible/provisioner.rb

@@ -150,7 +150,7 @@ module VagrantPlugins
           end
         end
 
-        return generated_inventory_file.to_s
+        return generated_inventory_dir.to_s
       end
 
       def get_extra_vars_argument