cbuildbot: enable Retries for Asynchronous Tests

Enable test-level HWTest retries by default for bvt-cq and asynchronous
tests.

BUG=chromium:533621
TEST=unittest

Change-Id: I30b3c71b17975bffd9c70ab009f925009235b877
Reviewed-on: https://chromium-review.googlesource.com/302135
Commit-Ready: Shuqian Zhao <shuqianz@chromium.org>
Tested-by: Shuqian Zhao <shuqianz@chromium.org>
Reviewed-by: Fang Deng <fdeng@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/304824
Commit-Queue: Shuqian Zhao <shuqianz@chromium.org>
3 files changed