Nova returns available extension list through its REST API.
The command of v2 API has been implemented, and this patch adds the
one of v3 API.
Note:
The command name of v2 is "nova list-extensions" but the one of v3,
which this patch adds, is "nova extension-list" because the name fits
to the other command names.
Change-Id: Ibee712b0aa5898a94e362f20be71e8863500b195