Fix doc generation for python 3
Fix doc generation for python 3

Fix the Sphinx html_last_updated_fmt for Python3. The
html_last_updated_fmt option is interpeted as a byte string in
python3, causing Sphinx build to break. This patch makes it
utf-8 string.

Change-Id: I399b769c3d636e40094837764da6a914132a779c

openstack/mistral-specs