Impement bulk polling of volumes

This implements Bulk volume polling using ideas presented by
justin in https://github.com/kubernetes/kubernetes/pull/39564

But it changes the implementation to use an interface
and doesn't affect other implementations.

pkg/volume/plugins.go

@@ -112,6 +112,11 @@ type VolumePlugin interface {
 	// Specifying mount options in a volume plugin that doesn't support
 	// user specified mount options will result in error creating persistent volumes
 	SupportsMountOption() bool
+
+	// SupportsBulkVolumeVerification checks if volume plugin type is capable
+	// of enabling bulk polling of all nodes. This can speed up verification of
+	// attached volumes by quite a bit, but underlying pluging must support it.
+	SupportsBulkVolumeVerification() bool
 }
 
 // PersistentVolumePlugin is an extended interface of VolumePlugin and is used