Workaround missing subprocess.check_output on Python 2.6

EL6 and derivatives can't use subprocess.check_output, since it
was not added until Python 2.7. Silently redirect them to the
existing ovirt.node.utils.process.pipe() function if they're on
Python 2.6

Change-Id: Ia201854707fd1300766488d8094585b90b868b3c
Signed-off-by: Ryan Barry <rbarry@redhat.com>