[autotest] Use stable version from afe_stable_versions table

Add a util to get stable version from afe_stable_versions table. If the table
is not configured, return the global config value CROS.stable_cros_version.

Add an RPC get_stable_version(board) to get stable version for given board.
If board is not specified, default stable version will be returned.

Update calls in moblab_RunSuite and cros_host to get stable version through
the new RPC.

BUG=chromium:436656
DEPLOY=apache
CQ-DEPEND=CL:236571
TEST=local rpc test, run repair, confirm repair uses stable version from
afe_stable_versions table.

Change-Id: I6a22a4b471060297403cc1ae7fcbf96bec6563d9
Reviewed-on: https://chromium-review.googlesource.com/236582
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
Trybot-Ready: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
6 files changed