[autotest] Optimize delay_minutes setting in suite scheduler.

We are seeing some odd behavior of suite scheduler that it schedule
suites for the same board repeatedly while failed to schedule suites for
some other boards. This may be related to some multithreading issue.
Though I haven't found the root cause, there are several issues we can
improve.

BUG=chromium:591538
TEST=unittest, local suite schedule run.

Change-Id: If79f518df38a1bdf91968c5e7eefa3640dfde27a
Reviewed-on: https://chromium-review.googlesource.com/330168
Reviewed-by: Dan Shi <dshi@google.com>
Commit-Queue: Dan Shi <dshi@google.com>
Trybot-Ready: Dan Shi <dshi@google.com>
Tested-by: Dan Shi <dshi@google.com>
diff --git a/site_utils/suite_scheduler/task.py b/site_utils/suite_scheduler/task.py
index 909b602..940cbba 100644
--- a/site_utils/suite_scheduler/task.py
+++ b/site_utils/suite_scheduler/task.py
@@ -801,13 +801,13 @@
         """
         test_source_build_msg = (
                 ' Test source build is %s.' % test_source_build
-                if test_source_build else None)
+                if test_source_build else '')
         firmware_rw_build_msg = (
                 ' Firmware RW build is %s.' % firmware_rw_build
-                if firmware_rw_build else None)
+                if firmware_rw_build else '')
         firmware_ro_build_msg = (
                 ' Firmware RO build is %s.' % firmware_ro_build
-                if firmware_ro_build else None)
+                if firmware_ro_build else '')
         build_string = cros_build or launch_control_build
         logging.debug('Schedule %s for build %s.%s%s%s',
                       self._suite, build_string, test_source_build_msg,
@@ -921,7 +921,8 @@
         logging.info('Running %s on %s', self._name, board)
         for build in launch_control_builds:
             try:
-                self._ScheduleSuite(scheduler, None, None, None, None,
+                self._ScheduleSuite(scheduler, None, None, None,
+                                    test_source_build=build,
                                     launch_control_build=build, board=board,
                                     force=force, run_prod_code=True)
             except deduping_scheduler.DedupingSchedulerException as e: