Add missing DefaultStorage interface

src/main/java/com/microsoft/azure/spark/tools/clusters/DefaultStorage.java

@@ -0,0 +1,10 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+package com.microsoft.azure.spark.tools.clusters;
+
+import com.microsoft.azure.spark.tools.job.Deployable;
+
+public interface DefaultStorage {
+    Deployable createDefaultDeployable();
+}