Mark playbook objects as finalized after post_validate

After post_validate() is called on an object, there should be no
need to continue looking up at parent attributes. This patch adds a
new flag (_finalized) which is set to True at the end of post_validate,
and getattr will not look beyond its own attributes from that point on.

lib/ansible/playbook/base.py

@@ -65,6 +65,9 @@ class Base:
     _ignore_errors       = FieldAttribute(isa='bool')
     _check_mode          = FieldAttribute(isa='bool')
 
+    # other internal params
+    _finalized           = False
+
     # param names which have been deprecated/removed
     DEPRECATED_ATTRIBUTES = [
         'sudo', 'sudo_user', 'sudo_pass', 'sudo_exe', 'sudo_flags',
@@ -118,7 +121,7 @@ class Base:
         except AttributeError:
             try:
                 value = self._attributes[prop_name]
-                if value is None:
+                if value is None and not self._finalized:
                     try:
                         if prop_name in self._cached_parent_attrs:
                             value = self._cached_parent_attrs[prop_name]
@@ -421,6 +424,8 @@ class Base:
                     raise AnsibleParserError("the field '%s' has an invalid value, which appears to include a variable that is undefined."
                             " The error was: %s" % (name,e), obj=self.get_ds())
 
+        self._finalized = True
+
     def serialize(self):
         '''
         Serializes the object derived from the base object into