Admin should see all flavors

Currently, after we create a new private flavor, we can't see it from nova flavor-list, But it's truly there. This is not convenience for administrator to maintain flavors. To avoid impact the original logical of flavor-list, we should add a new interface to list all flavors for admin user.

nova flaovr-list-all
GET /v2/:tenant_id/flavors/all

This is related to the horizon blueprint: https://blueprints.launchpad.net/horizon/+spec/define-flavor-for-project