Prevent deleting all deployed resources when GitTrack resources deleted

[sebastianrosch]

Accidentally or by any other means deleting the GitTrack resource would result in all deployed resources managed by Faros being deleted. As the default behaviour is --cascade=true, we consider this a high risk and would like to prevent this from happening (especially in production).

Proposal:
Config option (in GitTrack or cmd option) to prevent Faros from creating ownerReferences but use other means to identify the parent->child relationship between GitTrack and GitTrackObjects.

We have tested this with a faros.pusher.com/owner-reference annotation, which allows Faros to continue operating without the risk of using an ownerReference. Here is the code that I used to test this:

const ownerReferenceAnnotation = "faros.pusher.com/owner-reference"
const ownerReferenceAnnotationFormat = "%s.%s.%s/%s"
...

func (r *ReconcileGitTrack) listObjectsByName(owner *farosv1alpha1.GitTrack) (map[string]farosv1alpha1.GitTrackObjectInterface, error) {
...

    for _, gto := range gtos.Items {
	if farosflags.CreateOwnerReference {
	    if metav1.IsControlledBy(&gto, owner) {
		result[gto.GetNamespacedName()] = gto.DeepCopy()
	    }
	} else {
	    if isOwnedBy(&gto, owner) {
		result[gto.GetNamespacedName()] = gto.DeepCopy()
	    }
	}
    }
...
}

// isOwnedBy checks if the GitTrackObject is owned by owner by checking its annotations
func isOwnedBy(obj *farosv1alpha1.GitTrackObject, owner *farosv1alpha1.GitTrack) bool {
    annotations := obj.GetAnnotations()
    if annotations != nil {
	compare := fmt.Sprintf(ownerReferenceAnnotationFormat, owner.Name, owner.Kind, owner.TypeMeta.GroupVersionKind().Group, owner.GroupVersionKind().Version)
	if annotations[ownerReferenceAnnotation] == compare {
	    return true
	}
    }
    return false
}

...

func (r *ReconcileGitTrack) handleObject(u *unstructured.Unstructured, owner *farosv1alpha1.GitTrack) result {
...

    if farosflags.CreateOwnerReference {
        // Creating an owner reference, as before.
        if err = controllerutil.SetControllerReference(owner, gto, r.scheme); err != nil {
           return errorResult(gto.GetNamespacedName(), err)
        }
    } else {
        // Add an annotation to identify the Faros owner of this GitTrackObject
	annotations := gto.GetAnnotations()
	if annotations == nil {
	    annotations = make(map[string]string)
	}
	annotations[ownerReferenceAnnotation] = fmt.Sprintf(ownerReferenceAnnotationFormat, owner.Name, owner.Kind, owner.TypeMeta.GroupVersionKind().Group, owner.GroupVersionKind().Version)
	gto.SetAnnotations(annotations)
    }
...
}