GCE PD Detach fails if node no longer exists

Problem:

If a node with a GCE PD attached is deleted (before the volume is detached), subsequent attempts by the attach/detach controller to detach it continuously fail, and prevent the controller from attaching the volume to another node.

Repro steps:

Create a pod referencing a GCE PD
Wait for pod to get scheduled and running.
Delete the node VM (using gcloud) that the pod is scheduled to.
Check if volume is detached by attach/detach controller:
- Expected: Volume detached.
- Actual: Volume continuously fails detach.

Logs:

Details

From `/var/log/kube-controller-manager.log`:

I0721 03:21:43.464087       7 reconciler.go:134] Started DetachVolume for volume "X" from node "Y" due to maxWaitForUnmountDuration expiry.
E0721 03:21:43.591941       7 gce.go:2580] getInstanceByName/single-zone: failed to get instance Y; err: googleapi: Error 404: The resource 'projects/[project]/zones/[zone]/instances/Y' was not found, notFound
E0721 03:21:43.591985       7 attacher.go:215] Error checking if PD ("[pdname]") is already attached to current node ("Y"). Will continue and try detach anyway. err=instance not found
E0721 03:21:43.698786       7 gce.go:2580] getInstanceByName/single-zone: failed to get instance Y; err: googleapi: Error 404: The resource 'projects/[project]/zones/[zone]/instances/Y' was not found, notFound
E0721 03:21:43.698828       7 attacher.go:225] Error detaching PD "[pdname]" from node "Y": error getting instance "Y"

Workarounds:

Restart the kube-controller-manager binary on the master.

-or-

Recreate a node with the same name.

Proposed Fix:

If GCE PD detach fails with instance not found, assume successful detach.