Make list flavor show extra specs optional.

Flavor list will get each flavors extra_specs by making fresh requests to nova.
When there are lots of flavors, the flavor list will take a while to run. So
let us make show extra-specs optional.

Fix bug: #1166455

Change-Id: I86aef1035be6a88b8d9fb49a89f5a608a72589dd
Merge "Make list flavor show extra specs optional."