katib: StudyJob controller shouldn't crash if PyTorch (or other job operators not installed)

See: kubeflow/kubeflow#2212

User reports that the StudyJob controller crashes if pytorch operator isn’t installed on the cluster.

Logs for StudyJobController

$ kubectl logs -n kubeflow studyjob-controller-7d77f959-pjfzf
2019/01/06 22:43:34 Registering Components.
2019/01/06 22:43:34 controller.AddToManager(mgr)
2019/01/06 22:43:34 no matches for kind "PyTorchJob" in version "kubeflow.org/v1beta1"

This seems like a bug. If a particular job controller isn’t installed I would still expect katib and StudyJobs to work with other types of job controllers.