flavor-list --extra-specs need return the extra-specs information directly

Now the command of nova flavor-list --extra-specs can view
extra-specs information, but I use --debug to see a lot of
http GET requests for getting the extra_specs information
of each flavor.

With the increase of the flavors, it will get more and more
GET requests. This will affect the performance of the query.

I think that the query returns a list of flavor, it should
directly contain extra_specs information.

The part of debug info:
DEBUG (connectionpool:368) "GET /v2/ed952123e0cc4ced9e581a7710bc24d5/flavors/1/os-extra_specs HTTP/1.1" 200 66
DEBUG (session:224) RESP: [200] date: Thu, 01 Sep 2016 06:27:08 GMT connection: keep-alive content-type: application/json content-length: 66 x-compute-request-id: req-15182618-4b28-4c78-87ef-d51f8da309f3
RESP BODY: {"extra_specs": {}}

DEBUG (connectionpool:368) "GET /v2/ed952123e0cc4ced9e581a7710bc24d5/flavors/2/os-extra_specs HTTP/1.1" 200 19
DEBUG (session:224) RESP: [200] date: Thu, 01 Sep 2016 06:27:09 GMT connection: keep-alive content-type: application/json content-length: 19 x-compute-request-id: req-b519d74e-ed98-48e9-90be-838287f7e407
RESP BODY: {"extra_specs": {}}