We've had reliability issues with gerrit, primarily related to
429 status codes but also DDoS bans. The light DDoS bans can be
very short lived and we extend retries to handle this. Longer
term bans can take up to an hour to lift.

BUG=chromium:1071590
TEST=./gerrit_client_test.py

Change-Id: Iaf68c0d9cc7375aa58367ec0447d54a99f8ebf39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2153089
Commit-Queue: George Engelbrecht <engeg@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>