Port disk_config extension for V2.1

This patch ports v2 disk_config extension to v2.1.

Unittest code modified to share testing with both v2/v2.1.
API sample file and their tests have been added.

Also added extension point of server resize V3.

Partially implements blueprint v2-on-v3-api

Change-Id: Id2aace8121a310f8c86c6a9cc532bd1074d653c4
Merge "Port disk_config extension for V2.1"