Port user_data API to v3 Part 2

Ports the user_data extensions and the corresponding unittests
to the v3 framework. Moves the commented out tests from the v3
version of test_servers to the user data extension unittests.

Partially implements blueprint nova-v3-api

Change-Id: Id9b406271d821a99bbe7a3b3eb6ad7fc549ae7a3
Merge "Port user_data API to v3 Part 2"