ping_shopfloor: update registration message format

cid(client_id) has been changed to sid(session_id) in the recent version
of overlord.

BUG=none
TEST=manually run netboot install with overlord

Change-Id: Iff13026c0ce95a0fae7b7921b72ff43842ecd85c
Reviewed-on: https://chromium-review.googlesource.com/296250
Commit-Ready: Wei-Ning Huang <wnhuang@chromium.org>
Tested-by: Wei-Ning Huang <wnhuang@chromium.org>
Reviewed-by: Hsu Wei-Cheng <mojahsu@chromium.org>
1 file changed