autotest: Allow shill related scripts to run outside of autotest

Add a common.py and import common statements to fix up the import paths
in shill scripts.

TEST=`/usr/local/autotest/client/cros/scripts/wifi` displays usage
correctly when run.
BUG=chromium:328169

Change-Id: Ib155e1415138cf9d607d5df94b64ad1df0c1c566
Reviewed-on: https://chromium-review.googlesource.com/181930
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
5 files changed