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

Change-Id: I006b9cfef15a9e04c1e326d78dd30d2f5ad647c7
Closes-Bug:#1693670

stackforge/gce-api,stackforge/gce-api