Change `volume.config` to `volume.yaml`

[masaori335]

How about making it yaml like other configs?

Current

volume=1 scheme=http size=10%
volume=2 scheme=http size=1024

Proposal

• Add volumes as root key.
• Convert current tag to key of each entry.

volumes:
  - volume: 1
    scheme: http
    size: 10%
  - volume: 2
    scheme: http
    size: 1024