Fix doc generation for Python3
Fix doc generation for Python3

Fix the Sphinx html_last_updated_fmt for Python3.
The 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: Ia8274b7db7ff0f4e45acc778f7061c2add93c65f
Closes-Bug:#1693670

openstack/designate,openstack/designate,openstack/designate