Allow field attributes which are lists to validate the type of the list items

Starting to apply this for tags too, however it is not correcting things
as would be expected.

lib/ansible/playbook/attribute.py

@@ -21,12 +21,13 @@ __metaclass__ = type
 
 class Attribute:
 
-    def __init__(self, isa=None, private=False, default=None, required=False):
+    def __init__(self, isa=None, private=False, default=None, required=False, listof=None):
 
        self.isa = isa
        self.private = private
        self.default = default
        self.required = required
+       self.listof = listof
 
 class FieldAttribute(Attribute):
     pass