Register debusine workers

As a debusine administrator, shortly after having started a debusine worker on a new machine, I can login to the debusine web user interface and approve the addition of the new worker to the set of workers available to debusine. I can also do this approval through the debusine-admin command line tool. This lets me expand the computing resources available to debusine.

As a debusine administrator, I can edit arbitrary properties associated to each worker. Those properties can later be used to find appropriate workers for each task. The properties should likely be shown to me as arbitrary YAML data. I could for example set this:

sbuild:
  architectures:
    - amd64
    - i386
  distributions:
    - jessie
    - stretch

The worker is uniquely identified by a slug that is generated from the FQDN of the worker (adding -1, -2, -3 suffixes as required to avoid conflicts). The worker also submits its own unique token to authenticate itself. Once the worker has been approved, the token is enabled and the worker can do some API calls with it. The set of API available with this worker token is restricted. There's obviously an API to retrieve the next task to execute. That API might give a new task-specific token for further operations related to this task.