Add extension-list command for v3 API

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
Merge "Add extension-list command for v3 API"