CommitQueueCompletion and PublishUprevChanges stages are both git cleaning local changes. This kills the prebuild conf file commits (created by MasterUploadPrebuilts) prior to publish.

Solution: move the MUP logic inside of PUC, to run right after clean on master builds that publish prebuilts.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/cd0205aa67cd8ec6c1d98272ffb30a4ffea8f1ab

commit cd0205aa67cd8ec6c1d98272ffb30a4ffea8f1ab
Author: Aviv Keshet <akeshet@chromium.org>
Date: Thu Mar 03 17:36:06 2016

prebuilts: write binhost conf files in PublishUprev

This CL refactors MasterUploadPrebuiltsStage into a non-stage standalone
class. Instead of calling this stage prior to CompletionStage (as the CQ
currently does), the conf writing code is called in PublishUprev stage
immediately before uprev and conf commits are pushed.

This CL is a squashed version of the refactoring stack ending in
https://chromium-review.googlesource.com/#/c/330258/ plus any additional
post-refactor fixes.

BUG= chromium:591471 
TEST=cbuildbot --remote --buildbot --debug master-paladin with this CL

Change-Id: Iac3c8c9ceb6e491282e1c950c05c2718668e91b4
Reviewed-on: https://chromium-review.googlesource.com/330271
Commit-Ready: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>

[modify] https://crrev.com/cd0205aa67cd8ec6c1d98272ffb30a4ffea8f1ab/cbuildbot/builders/android_pfq_builders.py
[modify] https://crrev.com/cd0205aa67cd8ec6c1d98272ffb30a4ffea8f1ab/cbuildbot/stages/artifact_stages_unittest.py
[modify] https://crrev.com/cd0205aa67cd8ec6c1d98272ffb30a4ffea8f1ab/cbuildbot/prebuilts.py
[modify] https://crrev.com/cd0205aa67cd8ec6c1d98272ffb30a4ffea8f1ab/cbuildbot/stages/completion_stages.py
[modify] https://crrev.com/cd0205aa67cd8ec6c1d98272ffb30a4ffea8f1ab/cbuildbot/stages/completion_stages_unittest.py
[modify] https://crrev.com/cd0205aa67cd8ec6c1d98272ffb30a4ffea8f1ab/cbuildbot/prebuilts_unittest.py
[modify] https://crrev.com/cd0205aa67cd8ec6c1d98272ffb30a4ffea8f1ab/cbuildbot/builders/simple_builders.py
[modify] https://crrev.com/cd0205aa67cd8ec6c1d98272ffb30a4ffea8f1ab/cbuildbot/stages/artifact_stages.py

Bulk verified