Minor fix for script-without-shebang: MESOS-831.

From: Timothy St. Clair <tstclair@redhat.com>
Review: https://reviews.apache.org/r/15764
Added loadavg() convenience method to stout.

This patch includes a wrapper to get system load averages in uptime(1)
format. This is used by an upcoming patch which expose these averages
over master and slave stats.json endpoints.

Review: https://reviews.apache.org/r/15653