git_cl: Allow server overriding.
git_cl: Allow server overriding.

BUG=chromium:657064
TEST=local
  - Ran "git cl status" during outage with override, works.

R=tandrii@chromium.org

Review URL: https://chromiumcodereview-hr.appspot.com/2425673005 .

CoherentLabs/depot_tools,CoherentLabs/depot_tools