Port disk_config API to v3 Part 2

This patch contains the changes required to adapt the agent
extension and the corresponding unittest to the v3 framework.
Moves the commented out tests from the v3 version of test_servers
to the user data extension unittests.

Partially implements: bp v3-api-extension-versioning
Partially implements blueprint nova-v3-api

Change-Id: I7eef3ba18d7cc36c1ef78d093f50fd9122ac26c7
Merge "Port disk_config API to v3 Part 2"