Introduce StorageManager and an Hdfs Implementation for Storage

[HotSushi]

Summary

Laying foundations for storage part 3: StorageManager and an implementation for Hdfs: HdfsStorage.

StorageManager interface looks like

StorageManager {
  Storage getDefaultStorage()
  Storage getStorage(Type)
}

It runs through all configured Storages and returns the appropriate storage for default or for particular type.

HdfsStorage is yet another implementation for Storage similar to LocalStorage.
We've abstracted the common code for all storage-impls into BaseStorage

Changes

• New Features
  Other related PRs:
  Add cluster.yaml, StorageProperties, StorageType enum for supporting multiple storages #76
  Add interfaces for StorageClient, Storage and its Implementation for Local #82

Testing Done

• Added new tests for the changes made.

[abhisheknath2011]

Thanks @HotSushi . Added some comments.

[jainlavina]

Left a few comments. But looks good otherwise.

[abhisheknath2011]

LGTM. Thanks for addressing the comments/questions!