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