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.

Changing Popen to .check_output because of 2 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]

[1] https://review.openstack.org/#/c/457142
[2] https://review.openstack.org/#/c/433081

Change-Id: I810f841097c64c5e73db42ca393f6179182d9694

openstack/ceilometer-powervm,stackforge/ceilometer-powervm,stackforge/ceilometer-powervm,openstack/ceilometer-powervm