Improve registry config

[eriknelson]

Registries are produced via factory, but the RegistryConfig struct is currently trying to serve the needs of each, which means values that are required to be present in the registry section of the config are not used for some concrete registry adapters.

Need to get better with the types here, i.e.

  registry:
    type: dev
    fields:
       user: hello
       url: foo.bar.com

[eriknelson]

There are a finite number of adapter types; k8s uses Kind/Spec pattern to account for variable schemas, could use:

registry:
  kind: dockerhub
  spec:
     key: value
     dockerhub_specific_key: val

vs

registry:
  kind: otherkind
  spec:
     key: value
     otherkind_specific_key: val

[rthallisey]

That would be a nice solution. We approaching the point where reducing the amount of yaml is imperative.

[rthallisey]

#280