Sync oslo strutils.py

Update openstack/common/strutils.py, which pulls in these commits:
e53fe85 strutils bool_from_string, allow specified default
84d461e Fix a bug in safe_encode where it returns a bytes object in py3

Change-Id: I91d0dbaf40db57d37be170f12c2ef455f0aeb887
Merge "Sync oslo strutils.py"