[autotest] Don't schedule HWTests if less than 4 duts in the pool is available.

BUG=chromium:372085
TEST=unittest, test run_suite against different boards/pools in cautotest
Confirm run_suite returns warning if not enough duts available.

CQ-DEPEND=CL:198802

Change-Id: Icc2dd0adf2d9bb62da61e132ed04229d4f2bdc01
Reviewed-on: https://chromium-review.googlesource.com/201078
Commit-Queue: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Prashanth B <beeps@chromium.org>
3 files changed