Sync nullable attributes with Cluster API

[WilliamDEdwards]

The attributes that are marked as nullable and non-nullable are out of sync between the Cluster API and the client. Every attribute should be checked and updated if needed.

[dvdheiden]

To fully match the objects in the Cluster API, we could consider using the create/update models from the Cluster API. Right now, the models in this package represent the entity (i.e. VirtualHost) and are used for all CRUD actions.

That will be a breaking change, so let's discuss this first.

[WilliamDEdwards]

When doing the major version increase, change all references to Cluster API to Core API (what I wanted to do in #114).

[WilliamDEdwards]

Will be automatically fixed by #117