[autotest] Force AvailableHosts to consider testbed_dut_count

For testbed, the board label has a special format to tail '-[dut_count]' in
the board label name. That leads to an issue for suite scheduler to check dut
availability before running a task.

This change updates the AvailableHosts method so it use the right board label
for testbed.

BUG=b:31889351
TEST=local run suite_scheduler, unittest
make sure the dut has no label board:shamu, and run command
/usr/local/autotest/site_utils/suite_scheduler/suite_scheduler.py \
  -d /usr/local/autotest/logs -f /usr/local/autotest/test_ss.ini  \
  -e nightly  -i git_mnc-release/shamu-userdebug/2457013 \
  -r /tmp/_autotmp_0pjXWQ_suite_scheduler -o android

DEPLOY=suite_scheduler

Change-Id: I889249be9759142537ef7d7b69c14efa0a945967
Reviewed-on: https://chromium-review.googlesource.com/398260
Commit-Ready: Dan Shi <dshi@google.com>
Tested-by: Dan Shi <dshi@google.com>
Reviewed-by: Kevin Cheng <kevcheng@chromium.org>
diff --git a/site_utils/suite_scheduler/task.py b/site_utils/suite_scheduler/task.py
index 9897a7c..349adb3 100644
--- a/site_utils/suite_scheduler/task.py
+++ b/site_utils/suite_scheduler/task.py
@@ -817,7 +817,10 @@
         if self._boards and board not in self._boards:
             return []
 
-        labels = [Labels.BOARD_PREFIX + board]
+        board_label = Labels.BOARD_PREFIX + board
+        if self._testbed_dut_count:
+            board_label += '-%d' % self._testbed_dut_count
+        labels = [board_label]
         if self._pool:
             labels.append(Labels.POOL_PREFIX + self._pool)