cbuildbot: Use REEXEC_API for ts-mon-task-num.

Create REEXEC_API_TSMON_TASK_NUM, and use it to filter
ts-mon-task-num, including the option magically inserted.

BUG=chromium:740562
TEST=Unittests

Change-Id: Idee3b29d0c11dcd577891b6c2423a230d7fba1c1
Reviewed-on: https://chromium-review.googlesource.com/580356
Commit-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Paul Hobbs <phobbs@google.com>
diff --git a/scripts/cbuildbot_launch_unittest.py b/scripts/cbuildbot_launch_unittest.py
index 557b6e9..677eb5a 100644
--- a/scripts/cbuildbot_launch_unittest.py
+++ b/scripts/cbuildbot_launch_unittest.py
@@ -92,8 +92,7 @@
   """Tests for cbuildbot_launch script."""
 
   ARGS_BASE = ['--buildroot', '/buildroot']
-  EXPECTED_ARGS_BASE = ['--ts-mon-task-num', '1',
-                        '--buildroot', '/cbuildbot_buildroot']
+  EXPECTED_ARGS_BASE = ['--buildroot', '/cbuildbot_buildroot']
   ARGS_GIT_CACHE = ['--git-cache-dir', '/git-cache']
   ARGS_CONFIG = ['config']
   CMD = ['/cbuildbot_buildroot/chromite/bin/cbuildbot']
@@ -170,10 +169,12 @@
 
     # Ensure we invoke cbuildbot, as expected.
     self.assertCommandCalled(
-        ['/root/repository/chromite/bin/cbuildbot',
-         'config',
-         '--ts-mon-task-num', '1',
-         '-r', '/root/repository'],
+        [
+            '/root/repository/chromite/bin/cbuildbot',
+            'config',
+            '-r', '/root/repository',
+            '--ts-mon-task-num', '1',
+        ],
         cwd='/root/repository',
         error_code_ok=True)
 
@@ -221,13 +222,15 @@
 
     # Ensure we invoke cbuildbot, as expected.
     self.assertCommandCalled(
-        ['/root/repository/chromite/bin/cbuildbot',
-         'config',
-         '--ts-mon-task-num', '1',
-         '--buildroot', '/root/repository',
-         '--branch', 'branch',
-         '--git-cache-dir', '/git-cache',
-         '--remote-trybot'],
+        [
+            '/root/repository/chromite/bin/cbuildbot',
+            'config',
+            '--buildroot', '/root/repository',
+            '--branch', 'branch',
+            '--git-cache-dir', '/git-cache',
+            '--remote-trybot',
+            '--ts-mon-task-num', '1',
+        ],
         cwd='/root/repository',
         error_code_ok=True)