add basic bootstrap for coverage

PRESUBMIT.py

 #!/usr/bin/env python
 # Copyright 2015 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 import os
 import re
 import subprocess
 import sys
 
@@ skipping 19 matching lines @@
   def tests(*path):
     return input_api.canned_checks.GetUnitTestsInDirectory(
         input_api,
         output_api,
         input_api.os_path.join(*path),
         whitelist=[r'.+_test\.py'])
 
   results = []
 
   results.extend(input_api.canned_checks.PanProjectChecks(
-      input_api, output_api, license_header=header(input_api)))
+      input_api, output_api, license_header=header(input_api),
+      excluded_paths=['bootstrap/virtualenv/*']
+  ))
 
   results.extend(input_api.RunTests(
       tests('recipe_engine', 'unittests') +
       tests('unittests')))
 
   return results
 
 
 CheckChangeOnUpload = CommonChecks
 CheckChangeOnCommit = CommonChecks