Fix doc build with Python 2.6.x
Fix doc build with Python 2.6.x

subprocess.check_output was new in Python 2.7.
Use an alternative construct via subprocess.Popen
which works on Python 2.6 as well.

Change-Id: I0b44fc19183f1c6b23fe5a9cce31de381809534d

varunarya10/cliff,idjaw/cliff,idjaw/cliff,varunarya10/cliff,enzochiau/cliff,dtroyer/cliff,enzochiau/cliff,dtroyer/cliff,openstack/cliff,openstack/cliff