inform_shopfloor: retry until success

Retry until sucessfully informing shopfloor instead of stopping all process.

BUG=chrome-os-partner:50659
TEST=Manually on Chell.

Change-Id: I99d4d2eb4c1cae740bef13ef59a4332bb0441102
Reviewed-on: https://chromium-review.googlesource.com/334940
Commit-Ready: Shun-Hsing Ou <shunhsingou@chromium.org>
Tested-by: Shun-Hsing Ou <shunhsingou@chromium.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>