Fix update preview to handle nested stacks

Currently the update preview code has no support for previewing
the effect of an update on nested stacks, which I assume was an
oversight in the original implementation.  So this adds a show_nested
flag to the API which allows enabling recursive preview of the whole
update including nested stacks.

Closes-Bug: #1521971
Depends-On: I06f3b52d5d48dd5e6e266321e58ca8e6116d6017
Change-Id: I96af4d2f07056846aac7ae9ad9b6eb160e8bd51a
diff --git a/common/test.py b/common/test.py
index 1ffe222..8fc5c60 100644
--- a/common/test.py
+++ b/common/test.py
@@ -392,7 +392,8 @@
 
     def preview_update_stack(self, stack_identifier, template,
                              environment=None, files=None, parameters=None,
-                             tags=None, disable_rollback=True):
+                             tags=None, disable_rollback=True,
+                             show_nested=False):
         env = environment or {}
         env_files = files or {}
         parameters = parameters or {}
@@ -406,7 +407,8 @@
             disable_rollback=disable_rollback,
             parameters=parameters,
             environment=env,
-            tags=tags
+            tags=tags,
+            show_nested=show_nested
         )
 
     def assert_resource_is_a_stack(self, stack_identifier, res_name,