Port config_drive API to v3 Part 2

Ports the config_drive extensions and the corresponding unittests
to the v3 framework. Moves the commented out tests from the v3
version of test_servers to the config drive extension unittests.

Adds entrypoint infrastructure to allow extensions to extend the way
that the servers extension performs xml deserialization so extension
specific deserialization does not require changes to the core servers
extension. The blacklist/whitelist settings for extensions generally
also apply to server extension point loading.

Also merges in updates to config drive functionality which were applied
to the v2 API while this changeset has been in review

https://review.openstack.org/#/c/31314

Partially implements blueprint nova-v3-api

Change-Id: Ic8b75cc8b353a6e22cbaf354317a78c4df8d937e
Merge "Port config_drive API to v3 Part 2"