Enhance  V2 disk_config extension Unit Test

disk_config extension extends the request/response
for server's create, update, rebuild & resize operation.
However, unit tests of the extension are not enough now.

This patch adds more tests to increase code coverage.

Change-Id: Ibdc9a931fac7501a9f8326bae16f3b41d51be9bb
Merge "Enhance  V2 disk_config extension Unit Test"