[autotest] Support scheduling Launch Control builds from suite scheduler.

This change adds the support of scheduling Launch Control builds from
suite scheduler.

For task defined like:
[brollo_test]
run_on: nightly
hour: 17
suite: dummy
os_type: brillo
branches: git_mnc-brillo-dev
targets: dragonboard-userdebug
pool: suites

Suite scheduler will try to schedule the latest build as of
git_mnc-brillo-dev/dragonboard-userdebug/LATEST
for dragonbard daily at 5PM, to run a duts in suites pool.

BUG=chromium:585628
TEST=unittest, local suite scheduler run:
/usr/local/autotest/site_utils/suite_scheduler/suite_scheduler.py \
  -d /usr/local/autotest/logs -f /usr/local/autotest/ss_test.ini  \
  -e nightly -i git_mnc-brillo-dev/dragonboard-userdebug/2636090  \
  -r /tmp/_autotmp_0pjXWQ_suite_scheduler -o brillo

Also test existing cros build can be scheduled:
/usr/local/autotest/site_utils/suite_scheduler/suite_scheduler.py \
  -d /usr/local/autotest/logs -f /usr/local/autotest/ss_test.ini  \
  -e nightly  -i veyron_jerry-release/R49-7834.3.0  \
  -r /tmp/_autotmp_0pjXWQ_suite_scheduler

Change-Id: I6dc8ecb95ff0d353a1e8a96f98183945216f9a50
Reviewed-on: https://chromium-review.googlesource.com/329362
Commit-Ready: Dan Shi <dshi@google.com>
Tested-by: Dan Shi <dshi@google.com>
Reviewed-by: Dan Shi <dshi@google.com>
16 files changed