Kubelet: Add ContainerManager interface.

pkg/kubelet/container_manager.go

+/*
+Copyright 2015 The Kubernetes Authors All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package kubelet
+
+// Manages the containers running on a machine.
+type containerManager interface {
+	// Runs the container manager's housekeeping.
+	// - Ensures that the Docker daemon is in a container.
+	Start() error
+}