Once all branch configuration has been migrated to luci-config (issue 903420) and branch creation is automated (issue 904259), we should expose an API to create the associated branch builders. This can be as simple as template-driven submission of new config files to Gerrit or as complex as pushing new config files to the luci-config Push service. The following must be created:

* Scheduler files for the branch parent builder
* luci-config branch configuration (e.g. which child builders are expected)
* Any non-luci-config CrOS configuration that still lives in chromeos-config.py