`identityRef.kind` is ignored

[stephenfin]

/kind bug

What steps did you take and what happened:

Identified while working on fix for #1881. The identityRef field has a kind child field to allow you to specify the kind of object we inspect. Alas, it is completely ignored. We should either remove this field, make it an enum (["Secret"]), or otherwise not ignore it.

What did you expect to happen:

The field should do something.

Anything else you would like to add:

None.

Environment:

• Cluster API Provider OpenStack version (Or git rev-parse HEAD if manually built): master
• Cluster-API version: n/a
• OpenStack version: n/a
• Minikube/KIND version: n/a
• Kubernetes version (use kubectl version): n/a
• OS (e.g. from /etc/os-release): n/a

[mdbooth]

Lets make it an enum of ['Secret']. Lets also give it a default value of 'Secret' so it can be safely omitted.

The upgrade will be a bit weird. We need to ensure that objects with an invalid kind can be upgraded, and on upgrade the kind will be set to 'Secret'.

I think:
* Convert v1alphaN->v1alpha8 should unconditionally set the value to 'Secret'
* For v1alpha8->v1alphaN a restore function unconditionally restores its previous value

[mdbooth]

Actually the API guidelines have a specific section on this: https://github.com/kubernetes/community/blob/master/contributors/devel/sig-architecture/api-conventions.md#object-references.

Specifically we should probably follow the Single resource reference pattern omitting namespace as suggested. The guidelines note that this is an extensible pattern which can be upgraded to a Multiple resource reference in the future if required without breaking backwards compatibility.

So... I guess just delete the kind field.

[EmilienM]

/assign EmilienM