Oslo: sync strutils

The strutils module has been improved with regards to Python 3
compatibility.  For instance, safe_{encode,decode} may now take bytes
as input, which is needed to completely port heatclient to Python 3.

This module is now in sync with commit
c1f1a6d87d3f8a5420f5e749ae5ed85e15ff5fe4.

Change-Id: I658467dc85038da029e0ae8b40fc027cdc4d5eaf
Add stack-preview support

Co-Authored-By: Richard Lee <rblee88@gmail.com>
Change-Id: I4d7b12f6da09411d45dd6022dc11b8ece76b4932
Implements: blueprint preview-stack
Merge "Oslo: sync strutils"
Merge "Add stack-preview support"