fix the an Unexpected API Error issue in flavor API

The flavors.add_flavor_access and flavors.remove_flavor_access
require admin privilege in DB level but the policy doesn't
require it and the exception isn't catched in the api. So the
bug arise. The privilege check in DB level is not necessary,
but it need more tests and can be remove in other patch or
blue-print.

This use extension_authorizer instead of soft_extension_authorizer
in the action of flavor_access, in order to raise exception.

Closes-Bug: #1217679
DocImpact

Change-Id: I0b1231f47fffaf99f330de02956bfe8d7cd4b920
Merge "fix the an Unexpected API Error issue in flavor API"