Add storage option to load model (local / s3 / gcs / PV)

[Keigo Hattori]

Motivation

Current architecture requires that nodes must mount cloud storage. This spec is not developer-friendly.
Adding storage option will support developers to use Rekcurd easier.

Goal

• Get models via APIs.

Implementation

• Local access
• Use boto to connect ceph s3 APIs
• Use boto3 to connect AWS S3 APIs
• Use google-cloud-storage to connect GCS APIs
• testing