Import k8s.io/kubernetes/pkg/controller/volume/events

[anyasabo]

As part of #1723, I removed the k8s.io/kubernetes/pkg/controller/volume/events import where we were depending on the VolumeDelete constant defined there, but now I just copied it in. Per Jordan's comment here:
golang/go#32776 (comment)

k8s.io/kubernetes is not primarily intended to be consumed as a module.

But I did not see that constant defined anywhere else, but maybe I was missing it. It would be better if we could work out the imports rather than copy and pasting the constant value.

[racevedoo]

Hi @anyasabo, maybe copying is done by some other projects as well. I found a reference on https://godoc.org/github.com/kubernetes-incubator/external-storage/local-volume/provisioner/pkg/common saying exactly what you described here