Create a worker version without a revision

Depends #1624 (closed)
Refs https://redmine.teklia.com/issues/4471

An API user must be able to create a worker version, when they have admin level on that worker.

The CreateWorkerVersion endpoint must be updated to support this workflow:

• revision_id becomes optional
• revision_id can only be set if the worker has a repository set
• if revision_id is not set by the user:
  • the state can only be "available", raise a 400 if the user sets something else
  • docker_image must be set (to a docker image reference)