Depot tools now uses the project's cq.cfg by default on
git cl try, if no other method is specified.

Based on:
https://codereview.chromium.org/1579423004/
https://codereview.chromium.org/1587893003/

BUG=chromium:565952
TBR=tandrii@chromium.org
LOG=n
NOTRY=true

Review URL: https://codereview.chromium.org/1583233002

Cr-Commit-Position: refs/heads/master@{#33289}