Move storagedriver package to registry/storage/driver

This change is slightly more complex than previous package maves in that the
package name changed. To address this, we simply always reference the package
driver as storagedriver to avoid compatbility issues with existing code. While
unfortunate, this can be cleaned up over time.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/storage/driver/azure/blockblob.go

@@ -0,0 +1,24 @@
+package azure
+
+import (
+	"fmt"
+	"io"
+
+	azure "github.com/MSOpenTech/azure-sdk-for-go/clients/storage"
+)
+
+// azureBlockStorage is adaptor between azure.BlobStorageClient and
+// blockStorage interface.
+type azureBlockStorage struct {
+	azure.BlobStorageClient
+}
+
+func (b *azureBlockStorage) GetSectionReader(container, blob string, start, length int64) (io.ReadCloser, error) {
+	return b.BlobStorageClient.GetBlobRange(container, blob, fmt.Sprintf("%v-%v", start, start+length-1))
+}
+
+func newAzureBlockStorage(b azure.BlobStorageClient) azureBlockStorage {
+	a := azureBlockStorage{}
+	a.BlobStorageClient = b
+	return a
+}