Autogenerate CustomResourceDefinition instance from Model

[mortenlj]

When working with CRDs, the current design requires that you define your Model for working with objects of that CRD, but it also requires that you manually build a CustomResourceDefinition that you can save to the api-server to create your type to start with.

Much of the information used in CustomResourceDefinition is also used in the URLs needed in the models Meta.

It would make things smoother to work with if we extend Meta so that it also describes the fields in CRD. When present, MetaModel will add a method to create a CustomResourceDefinition object, ready for saving to the api-server.

This could also auto generate the URLs in Meta for this type, and the apiVersion and kind fields which are required on custom models today.

[mortenlj]

Additional improvement would be to add a cli mode for generating a yaml for the CRD.