Right now when a master wants to clean up obsolete slave builds, it searches with tag:build_type in chromeos and chromiumos buckets. 

It brings problems:

1) The master build in chromeos.release waterfall also tries to cancel the slave builds in chromeos and chromiumos waterfalls.
2) We can run 3 master builds on chromeos.relase waterfall on different branches at the same time. R57 branch master shouldn't cancel the slaves of R56 branch master. 

dgarrett@ mentioned there would be a plan to launch multiple android-pfq masters. This bug should be fixed before the multiple masters. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/911af741692fa4b9ec26bda1a48263ba4d2ec046

commit 911af741692fa4b9ec26bda1a48263ba4d2ec046
Author: Ningning Xia <nxia@chromium.org>
Date: Fri Feb 03 13:15:09 2017

Search obsolete slaves with right buckets and tags in CleanUpStage.

1) Add manifest branch to the slave build Buildbucket tag set.
2) When searching for obsolete slave builds to cancel:
 a) search in the right Buildbucket buckets.
 b) search with the right build tag.

BUG= chromium:684936 
TEST=unit_tests

Change-Id: I0b38b404fbd1af45238dd59633aac4c53304b2e7
Reviewed-on: https://chromium-review.googlesource.com/432998
Commit-Ready: Ningning Xia <nxia@chromium.org>
Tested-by: Ningning Xia <nxia@chromium.org>
Reviewed-by: Ningning Xia <nxia@chromium.org>

[modify] https://crrev.com/911af741692fa4b9ec26bda1a48263ba4d2ec046/cbuildbot/stages/scheduler_stages.py
[modify] https://crrev.com/911af741692fa4b9ec26bda1a48263ba4d2ec046/cbuildbot/stages/build_stages.py
[modify] https://crrev.com/911af741692fa4b9ec26bda1a48263ba4d2ec046/cbuildbot/stages/build_stages_unittest.py