Policy check returns HTTP status instead of JSON

[Thai Tran]

Policy check today returns a JSON object containing an allowed flag that can either be true or false. This requires that we check the response object for the flag. It should instead just return a 204 for allowed, or a 406 for not allowed (401 is already taken for unauthorized - which redirects a user to the logout screen). This is undesirable since we may want to hide content but not kick the user out.

We have future plans to batch policy checks and cache them, but for now, the plan is for check to do a singular policy check that expects a boolean. This is also more inline with the plans we have for hz-if-policies directive.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/217982

[Thai Tran (tqtran)]

Since there doesn't seem to be an HTTP code for no, we are sort of misusing 406. It is better to just write a wrapper for this service, bug is no longer valid.