Fix html_last_updated_fmt for Python3.
Fix html_last_updated_fmt for Python3.

html_last_updated_fmt option is interpreted as a
byte string in python3, causing Sphinx build to break.
This patch makes it utf-8 string.

Change-Id: If9104f4b56cd22fed87c596f990350ea877d83b6
Closes-Bug:#1693670

masayukig/tempest,masayukig/tempest,Juniper/tempest,cisco-openstack/tempest,Juniper/tempest,openstack/tempest,cisco-openstack/tempest,vedujoshi/tempest,openstack/tempest,vedujoshi/tempest