Comment 5 for bug 1447084

Describe more about the problem:

Before https://review.openstack.org/#/c/150568, user can update the policy rule for hypervisor api to "compute_extension:hypervisors": "", then it make the non-admin user can access the api.

But after that patch, it introduce a db call service_get_by_host_and_topic which required admin context. Then even the policy allow any user access the api, but the db call always stop that.

We should fix the API's behavior as before.