scheduler_stages: Fix release waterfall builder names.

If a release waterfall builder is run with a different branch (such as
a stabilize branch), the wrong builder name is generated for the slave
builders when scheduling them.

BUG=chromium:793447
TEST=run_tests

Change-Id: If19f5f0db2b457ed92aca5aa97573e8789eae162
Reviewed-on: https://chromium-review.googlesource.com/818307
Tested-by: Don Garrett <dgarrett@chromium.org>
Trybot-Ready: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Paul Hobbs <phobbs@google.com>
(cherry picked from commit 3043e79ef8e089b7627b1baa1b7df678701f1ee4)
Reviewed-on: https://chromium-review.googlesource.com/820650
Reviewed-by: Don Garrett <dgarrett@chromium.org>
2 files changed