bug 1415391 - move release indexes into index_builder('release'). r=dustin

In bug 1412690, Dustin noted that the scopes and routes don't belong at the worker level.
To deal with the release indexes, we now have a new `index_builder`. We also add the default
release bbb scope in `build_buildbot_bridge_payload`.

We can and should still move the product info to attributes. I left that for another patch.

MozReview-Commit-ID: 4ZqvnY577S7

taskcluster/taskgraph/transforms/beetmover_cdns.py

@@ -40,6 +40,8 @@ beetmover_cdns_description_schema = Schema({
         {'by-project': {basestring: job_description_schema['worker-type']}},
     ),
     Optional('dependencies'): {basestring: taskref_or_string},
+    Optional('index'): {basestring: basestring},
+    Optional('routes'): [basestring],
 })