We should have access to #core_client, #rbac_client,
and #extensions_client without having to pass in an awkward array.

Also change api_version to default_api_version, which allows us to use a
different version for an individual client. Special case for
apis/extensions which only go up to v1beta1

Makes #hashed_client private

Removes the #clients and #discover! methods which are un-used