Fix regression from 3c979cb54e which broke run_on_bots.py.
Fix regression from 3c979cb54e which broke run_on_bots.py.

Convert the assert to not fire up.

R=vadimsh@chromium.org
BUG=chromium:466751

Review URL: https://codereview.appspot.com/217870043

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py