Provide a REST endpoint interface to `register_start`

Interactions with comet are currently only possible using either the kotekan dataset broker or the python API. This effectively prevents us from writing external systems (brokers, etc...) in anything other than python, and forces a `comet` dependency on python code for nothing more than a single function call at startup.

It would be nice to be able to register a startup time and config via a broker endpoint. This would likely require moving the registration logic out of `Manager.regsiter_start` into the broker, and reworking `Manager.register_start` to simply jsonify its input and send that to the broker.

This might require a bit of additional thought, since it does slightly break the responsibility scope of the broker and manager.