Implements a credentialprovider library for use by DockerPuller.

This change refactors the way Kubelet's DockerPuller handles the docker config credentials to utilize a new credentialprovider library.

The credentialprovider library is based on several of the files from the Kubelet's dockertools directory, but supports a new pluggable model for retrieving a .dockercfg-compatible JSON blob with credentials.

With this change, the Kubelet will lazily ask for the docker config from a set of DockerConfigProvider extensions each time it needs a credential.

This change provides common implementations of DockerConfigProvider for:
 - "Default": load .dockercfg from disk
 - "Caching": wraps another provider in a cache that expires after a pre-specified lifetime.

GCP-only:
 - "google-dockercfg": reads a .dockercfg from a GCE instance's metadata
 - "google-dockercfg-url": reads a .dockercfg from a URL specified in a GCE instance's metadata.
 - "google-container-registry": reads an access token from GCE metadata into a password field.

cmd/kubelet/plugins.go

@@ -0,0 +1,24 @@
+/*
+Copyright 2014 Google Inc. 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 main
+
+// This file exists to force the desired plugin implementations to be linked.
+// This should probably be part of some configuration fed into the build for a
+// given binary target.
+import (
+	_ "github.com/GoogleCloudPlatform/kubernetes/pkg/credentialprovider/gcp"
+)