subnet: handle unknown state in delete event

To fix panic (see log below), add a handler for
cache.DeletedFinalStateUnknown.

```
/usr/local/go/src/runtime/asm_amd64.s:2197
panic: interface conversion: interface {} is cache.DeletedFinalStateUnknown, not *v1.Node [recovered]
	panic: interface conversion: interface {} is cache.DeletedFinalStateUnknown, not *v1.Node [recovered]
	panic: interface conversion: interface {} is cache.DeletedFinalStateUnknown, not *v1.Node

goroutine 19 [running]:
github.com/coreos/flannel/vendor/k8s.io/apimachinery/pkg/util/runtime.HandleCrash(0x0, 0x0, 0x0)
	/go/src/github.com/coreos/flannel/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:58 +0x126
panic(0x1782360, 0xc420945e80)
	/usr/local/go/src/runtime/panic.go:489 +0x2cf
github.com/coreos/flannel/vendor/k8s.io/apimachinery/pkg/util/runtime.HandleCrash(0x0, 0x0, 0x0)
	/go/src/github.com/coreos/flannel/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:58 +0x126
panic(0x1782360, 0xc420945e80)
	/usr/local/go/src/runtime/panic.go:489 +0x2cf
github.com/coreos/flannel/subnet/kube.(*kubeSubnetManager).handleAddLeaseEvent(0xc420014f00, 0x1, 0x17e38a0, 0xc4204f61c0)
	/go/src/github.com/coreos/flannel/subnet/kube/kube.go:176 +0x2d3
github.com/coreos/flannel/subnet/kube.newKubeSubnetManager.func4(0x17e38a0, 0xc4204f61c0)
	/go/src/github.com/coreos/flannel/subnet/kube/kube.go:165 +0x47
github.com/coreos/flannel/vendor/k8s.io/client-go/tools/cache.ResourceEventHandlerFuncs.OnDelete(0xc4202ba6e0, 0xc4202ba6f0, 0xc4202ba700, 0x17e38a0, 0xc4204f61c0)
	/go/src/github.com/coreos/flannel/vendor/k8s.io/client-go/tools/cache/controller.go:206 +0x49
github.com/coreos/flannel/vendor/k8s.io/client-go/tools/cache.(*ResourceEventHandlerFuncs).OnDelete(0xc42018c780, 0x17e38a0, 0xc4204f61c0)
	<autogenerated>:56 +0x73
github.com/coreos/flannel/vendor/k8s.io/client-go/tools/cache.NewIndexerInformer.func1(0x1792520, 0xc4204f6680, 0x1792520, 0xc4204f6680)
	/go/src/github.com/coreos/flannel/vendor/k8s.io/client-go/tools/cache/controller.go:342 +0x519
github.com/coreos/flannel/vendor/k8s.io/client-go/tools/cache.(*DeltaFIFO).Pop(0xc4203120c0, 0xc4203aede0, 0x0, 0x0, 0x0, 0x0)
	/go/src/github.com/coreos/flannel/vendor/k8s.io/client-go/tools/cache/delta_fifo.go:451 +0x27e
github.com/coreos/flannel/vendor/k8s.io/client-go/tools/cache.(*controller).processLoop(0xc420067180)
	/go/src/github.com/coreos/flannel/vendor/k8s.io/client-go/tools/cache/controller.go:147 +0x40
github.com/coreos/flannel/vendor/k8s.io/client-go/tools/cache.(*controller).(github.com/coreos/flannel/vendor/k8s.io/client-go/tools/cache.processLoop)-fm()
	/go/src/github.com/coreos/flannel/vendor/k8s.io/client-go/tools/cache/controller.go:121 +0x2a
github.com/coreos/flannel/vendor/k8s.io/apimachinery/pkg/util/wait.JitterUntil.func1(0xc4205fcf70)
	/go/src/github.com/coreos/flannel/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:96 +0x5e
github.com/coreos/flannel/vendor/k8s.io/apimachinery/pkg/util/wait.JitterUntil(0xc420865f70, 0x3b9aca00, 0x0, 0x166ac01, 0x0)
	/go/src/github.com/coreos/flannel/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:97 +0xad
github.com/coreos/flannel/vendor/k8s.io/apimachinery/pkg/util/wait.Until(0xc4205fcf70, 0x3b9aca00, 0x0)
	/go/src/github.com/coreos/flannel/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:52 +0x4d
github.com/coreos/flannel/vendor/k8s.io/client-go/tools/cache.(*controller).Run(0xc420067180, 0x0)
	/go/src/github.com/coreos/flannel/vendor/k8s.io/client-go/tools/cache/controller.go:121 +0x237
github.com/coreos/flannel/subnet/kube.(*kubeSubnetManager).Run(0xc420014f00, 0x7f340ee4d3b0, 0xc4200161a8)
	/go/src/github.com/coreos/flannel/subnet/kube/kube.go:284 +0x8a
created by github.com/coreos/flannel/subnet/kube.NewSubnetManager
	/go/src/github.com/coreos/flannel/subnet/kube/kube.go:128 +0x592
```

Signed-off-by: Peng Gao <peng.gao.dut@gmail.com>

subnet/kube/kube.go

@@ -163,6 +163,21 @@ func newKubeSubnetManager(c clientset.Interface, sc *subnet.Config, nodeName str
 			},
 			UpdateFunc: ksm.handleUpdateLeaseEvent,
 			DeleteFunc: func(obj interface{}) {
+				node, isNode := obj.(*v1.Node)
+				// We can get DeletedFinalStateUnknown instead of *api.Node here and we need to handle that correctly.
+				if !isNode {
+					deletedState, ok := obj.(cache.DeletedFinalStateUnknown)
+					if !ok {
+						glog.Infof("Error received unexpected object: %v", obj)
+						return
+					}
+					node, ok = deletedState.Obj.(*v1.Node)
+					if !ok {
+						glog.Infof("Error deletedFinalStateUnknown contained non-Node object: %v", deletedState.Obj)
+						return
+					}
+					obj = node
+				}
 				ksm.handleAddLeaseEvent(subnet.EventRemoved, obj)
 			},
 		},