Fix html_last_updated_fmt for Python3
Fix html_last_updated_fmt for Python3

gitsha  and pwd options are interpreted as a
byte string in python3, causing Sphinx build to break.
This patch makes it utf-8 string.

Change-Id: I9541793b4e979ce7637ee5a0e61a881826b3f86f

openstack/barbican,openstack/barbican