Replace while with if in resolve function

Functions can't return functions, so we don't need to use `while`.
This doesn't change behaviour of code, just makes it more understandable
and convenient.

Change-Id: Ia1dac5a9583cb99db23a8838412e5bc02e93ef4c

heat/engine/function.py

@@ -127,8 +127,8 @@ class Function(object):
 
 
 def resolve(snippet):
-    while isinstance(snippet, Function):
-        snippet = snippet.result()
+    if isinstance(snippet, Function):
+        return snippet.result()
 
     if isinstance(snippet, collections.Mapping):
         return dict((k, resolve(v)) for k, v in snippet.items())

heat/engine/resources/openstack/heat/resource_group.py

@@ -462,9 +462,9 @@ class ResourceGroup(stack_resource.StackResource):
         # assigned. Pass in a custom resolver to the properties to not
         # error when a parameter does not have a user entered value.
         def ignore_param_resolve(snippet):
-            while isinstance(snippet, function.Function):
+            if isinstance(snippet, function.Function):
                 try:
-                    snippet = snippet.result()
+                    return snippet.result()
                 except exception.UserParameterMissing:
                     return None