Scheduler should expose API to configure and view status without UI

[floriangrundig]

Please update the documentation how to use the api... If necessary implement the remaining features:

TODO: What should be exposed to the API?

[jhftrifork]

@mwl the "view status" part of this ticket might be useful. Technically all our state is stored in ZK, so the user could look there, but we could expose an /api/state on the scheduler which is essentially a proxy to the ZK state.