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 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: Ifeff184b71a83b78de751dfd0c2f43a33cd409ee

mahak/neutron,noironetworks/neutron,openstack/neutron,noironetworks/neutron,huntxu/neutron,openstack/neutron,huntxu/neutron,openstack/neutron,mahak/neutron,mahak/neutron,eayunstack/neutron,eayunstack/neutron