Adds initial volumes package; Supports host-dirs

Adds the framework for external volume mounts.

Currently supports bare host directory mounts.

Modifies the API to support host directory mounts from Volumes
instead of VolumeMounts.

pkg/volume/volume.go

@@ -0,0 +1,65 @@
+/*
+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 volumes
+
+import (
+	"errors"
+	"github.com/GoogleCloudPlatform/kubernetes/pkg/api"
+)
+
+
+// All volume types are expected to implement this interface
+type Interface interface {
+	// Prepares and mounts/unpacks the volume to a directory path.
+	SetUp()
+	// Returns the directory path the volume is mounted to.
+	GetPath() string
+	// Unmounts the volume and removes traces of the SetUp procedure.
+	TearDown()
+}
+
+// Host Directory Volumes represent a bare host directory mount.
+type HostDirectoryVolume struct {
+	Path string
+}
+
+// Simple host directory mounts require no setup or cleanup, but still
+// need to fulfill the interface definitions.
+func (hostVol *HostDirectoryVolume) SetUp() {}
+
+func (hostVol *HostDirectoryVolume) TearDown() {}
+
+func (hostVol *HostDirectoryVolume) GetPath() string {
+	return hostVol.Path
+}
+
+// Interprets API volume as a HostDirectory
+func createHostDirectoryVolume(volume *api.Volume) *HostDirectoryVolume {
+	return &HostDirectoryVolume{volume.HostDirectory.Path}
+}
+
+// Interprets parameters passed in the API as an internal structure
+// with utility procedures for mounting.
+func CreateVolume(volume *api.Volume) (Interface, error) {
+	// TODO(jonesdl) We should probably not check every
+	// pointer and directly resolve these types instead.
+	if volume.HostDirectory != nil {
+		return createHostDirectoryVolume(volume), nil
+	} else {
+		return nil, errors.New("Unsupported volume type.")
+	}
+}