Let bot_update recipe api force usage of bot_update.

scripts/slave/recipe_modules/bot_update/example.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 DEPS = [
   'bot_update',
   'gclient',
   'path',
   'properties',
 ]
 
 def GenSteps(api):
   api.gclient.use_mirror = True
 
   src_cfg = api.gclient.make_config()
   soln = src_cfg.solutions.add()
   soln.name = 'src'
   soln.url = 'svn://svn.chromium.org/chrome/trunk/src'
   api.gclient.c = src_cfg
-  yield api.bot_update.ensure_checkout()
+  force = True if api.properties.get('force') else False
+  yield api.bot_update.ensure_checkout(force=force)
 
 
 def GenTests(api):
   yield api.test('basic') + api.properties(
       mastername='chromium.linux',
       buildername='Linux Builder',
       slavename='totallyaslave-m1',
   )
   yield api.test('tryjob') + api.properties(
       mastername='tryserver.chromium',
       buildername='linux_rel',
       slavename='totallyaslave-c4',
       issue=12345,
       patchset=654321,
       patch_url='http://src.chromium.org/foo/bar'
   )
+  yield api.test('forced') + api.properties(
+      mastername='experimental',
+      buildername='Experimental Builder',
+      slavename='somehost',
+      force=1
+  )
+  yield api.test('off') + api.properties(
+      mastername='experimental',
+      buildername='Experimental Builder',
+      slavename='somehost',
+  )