Fix Error: attribution() takes exactly 3 arguments (2 given)

Commit 3ab0be1f3e introduced some
DSL reworking, but in once case where attribution() was called, it
hadn't been updated.

This commit adds a missing parameter to the function, which appears
to fix the issue.

Change-Id: I9319430af002de3d85e74b9713624b7b2ea1ab2f
Closes-Bug: #1776601

murano/dsl/lhs_expression.py

@@ -111,7 +111,7 @@ def _prepare_context():
             if utils.is_sequence(src):
                 src_property.set(src[:index] + (value,) + src[index + 1:])
             elif isinstance(src, utils.MappingType):
-                attribution(src_property, index).set(value)
+                attribution(context, src_property, index).set(value)
 
         if isinstance(index, int):
             return _Property(