Teach cbuildbot to retry the VMTest stage.

BUG=chromium:309041, chromium:254166
TEST=Add flake, verify it retries.
TEST=Run all unit tests.

Change-Id: I49c6ec5cb8255c1d664100a6562959cf1e9aad97
Reviewed-on: https://chromium-review.googlesource.com/173751
Reviewed-by: David James <davidjames@chromium.org>
Tested-by: David James <davidjames@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: David James <davidjames@chromium.org>
7 files changed