Migrate kubelet configmap management logic to an interface

pkg/volume/plugins.go

@@ -233,6 +233,9 @@ type VolumeHost interface {
 	// Returns a function that returns a secret.
 	GetSecretFunc() func(namespace, name string) (*v1.Secret, error)
 
+	// Returns a function that returns a configmap.
+	GetConfigMapFunc() func(namespace, name string) (*v1.ConfigMap, error)
+
 	// Returns the labels on the node
 	GetNodeLabels() (map[string]string, error)
 }