quick-provision: Handle ssh errors more gracefully.

There is a race upon reboot of the quick provision pass where the reboot
can occur and terminate the ssh connection resulting in an ssh failure.
Capture that failure and use the COMPLETED keyval to determine if the
error should be ignored.

BUG=chromium:817955
TEST=~autotest/contrib/loadtest.py DEVSERVER config-cyan.json -t 1 -s 1 --ping

Change-Id: I8f633b37581580483dce9970525857d1e828c229
Reviewed-on: https://chromium-review.googlesource.com/944564
Commit-Ready: David Riley <davidriley@chromium.org>
Tested-by: David Riley <davidriley@chromium.org>
Reviewed-by: Congbin Guo <guocb@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
2 files changed