avoid inheritance issues with default=dict declaration at class level

this should avoid the issue of subsequent plays not prompting for a var
prompted for in a previous play.

lib/ansible/playbook/base.py

@@ -49,7 +49,7 @@ class Base:
     _remote_user         = FieldAttribute(isa='string')
 
     # variables
-    _vars                = FieldAttribute(isa='dict', default=dict(), priority=100)
+    _vars                = FieldAttribute(isa='dict', priority=100)
 
     # flags and misc. settings
     _environment         = FieldAttribute(isa='list')
@@ -77,6 +77,9 @@ class Base:
         # and initialize the base attributes
         self._initialize_base_attributes()
 
+        if self.vars is None:
+            self.vars = dict()
+
     # The following three functions are used to programatically define data
     # descriptors (aka properties) for the Attributes of all of the playbook
     # objects (tasks, blocks, plays, etc).