[autotest] Schedule tests against lumpy-pgo-release builds correctly.

I now understand how this code should work.  For the moment, I'm going
to treat pgo-release as a bare branch.  If the build team ever decides
that they want to do pgo-release across multiple versions, there's going
to be some suite_scheduler rework needed to be able to specify
milestones on non-release branches.

With this change, we will not schedule suites on lumpy-pgo-release by
default, but if one does want to start scheduling against pgo-release
builds, one can add a stanza to suite_scheduler.ini with
|branch_specs: pgo-release| in the stanza.

BUG=chromium-os:39283
TEST=unit, forced dummy suite on lumpy-pgo-release

Change-Id: I8042a5788cfa58333402cd2660be989c6ea67800
Reviewed-on: https://gerrit.chromium.org/gerrit/44874
Tested-by: Alex Miller <milleral@chromium.org>
Reviewed-by: Simran Basi <sbasi@chromium.org>
Commit-Queue: Alex Miller <milleral@chromium.org>
diff --git a/site_utils/suite_scheduler/driver.py b/site_utils/suite_scheduler/driver.py
index a28ca4f..aa813b7 100644
--- a/site_utils/suite_scheduler/driver.py
+++ b/site_utils/suite_scheduler/driver.py
@@ -154,7 +154,7 @@
         """
         board, type, milestone, manifest = base_event.ParseBuildName(build_name)
         branch_builds = {task.PickBranchName(type, milestone): [build_name]}
-        logging.info('Testing build %s-%s on %s', milestone, manifest, board)
+        logging.info('Testing build R%s-%s on %s', milestone, manifest, board)
 
         for e in self._events.itervalues():
             if e.keyword in keywords: