Add API validation schema for disk_config

This patch adds API validation schema for disk_config.

This patch also modify TestServersExtensionSchema to load
disk_config extension so that update, rebuild & resize
extension schema loading can be verified.

Partially implements blueprint v2-on-v3-api

Change-Id: Ib687a8e4e8b173c32bcd19e396b23fbf2f3319c6
Merge "Add API validation schema for disk_config"