Report out of disk as a node condition when node goes out of disk.

Define a new out of disk node condition and use it to report when node
goes out of disk.

Make a copy of loop range clause variable in node listers so that it
is available outside the for loop.

Also update/implement unit tests.

pkg/api/types.go

@@ -1491,6 +1491,9 @@ type NodeConditionType string
 const (
 	// NodeReady means kubelet is healthy and ready to accept pods.
 	NodeReady NodeConditionType = "Ready"
+	// NodeOutOfDisk means the kubelet will not accept new pods due to insufficient free disk
+	// space on the node.
+	NodeOutOfDisk NodeConditionType = "OutOfDisk"
 )
 
 type NodeCondition struct {