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.
Changing Popen to .check_output because of 3 reasons:
1. check_output() will raise CalledProcessError if
the called process returns a non-zero return code.
2. For consistency with keystone [1] and cinder [2]
3. It makes the code look much better.
[1] https://review.openstack.org/#/c/457142/
[2] https://review.openstack.org/#/c/433081
Change-Id: I8872866338d93bc8328391537198df6c7981859f
d94cdf8596