Make ask_mapping a simple class property

from PR feedback of saved launchtime configurations

awx/main/fields.py

@@ -767,6 +767,16 @@ class AskForField(models.BooleanField):
     """
     Denotes whether to prompt on launch for another field on the same template
     """
-    def __init__(self, allows_field='__default__', **kwargs):
+    def __init__(self, allows_field=None, **kwargs):
         super(AskForField, self).__init__(**kwargs)
-        self.allows_field = allows_field
+        self._allows_field = allows_field
+
+    @property
+    def allows_field(self):
+        if self._allows_field is None:
+            try:
+                return self.name[len('ask_'):-len('_on_launch')]
+            except AttributeError:
+                # self.name will be set by the model metaclass, not this field
+                raise Exception('Corresponding allows_field cannot be accessed until model is initialized.')
+        return self._allows_field