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: Ic85b544a5d91f0f820737480fd123cbebdc94894

openstack/magnum-ui,openstack/magnum-ui,openstack/magnum-ui,openstack/magnum-ui