Alphabetize core list for V3 API plugins

Alphabetize list and one line per plugin for future managability.
Deferred from a review comment on a previous changeset:

https://review.openstack.org/#/c/53104/3/nova/api/openstack/__init__.py

Partially implements blueprint nova-v3-api

Change-Id: Iedfae644b97369cc9694df7a10444311418fffd0
Merge "Alphabetize core list for V3 API plugins"