Fixed broken assignment to dictionary using integer key

The case

$dict[$key]: value

for integer $key was not addressed in LHS expression logic.
As a result no changes were made to $dict and no error was raised.

Change-Id: I969dab5f0d32312cf52490945934557c32f22dde
Closes-Bug: #1515612

murano/dsl/lhs_expression.py

@@ -107,6 +107,8 @@ class LhsExpression(object):
                 src = src_property.get()
                 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)
 
             if isinstance(index, int):
                 return LhsExpression.Property(