attribute defaults that are containers are a copy

This is simpler way to prevent persistent containers across instances
of classes that use field attributes

lib/ansible/playbook/attribute.py

@@ -19,6 +19,7 @@
 from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
+from copy import deepcopy
 
 class Attribute:
 
@@ -32,6 +33,11 @@ class Attribute:
         self.priority = priority
         self.always_post_validate = always_post_validate
 
+        if default is not None and self.isa in ('list', 'dict', 'set'):
+            self.default = deepcopy(default)
+        else:
+            self.default = default
+
     def __eq__(self, other):
         return other.priority == self.priority