Allow more fine grained authorization check in swift (keystoneauth).

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Currently in swift, the authorization checks that are defined in keystoneauth middleware are based on 3 main kind of user profiles:
* "anonymous" users (users that are not authenticated), and that can only perform some specific actions (public actions)
* "operators" (users that have one of the role defined in the parameter operator_roles), they have almost the right of doing everything as long as it's in their own tenant.
* "resellers admins" (users that have the role defined in the parameter reseller_admin_role), they have the right of doing almost everything.

If this profiles can handle many use cases, some users may still need a more fine grained profile/authorization checks.
For example we may need to define a profile only capable of listing objects/containers and not able to upload/create new objects, etc.

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For this we propose to implement a policy engine within keystoneauth middleware.
The same way other openstack components already implement through the json policy format (based on https://github.com/openstack/oslo-incubator/blob/master/openstack/common/policy.py).

This way, it would be possible to define user specific authorizations for different kind of profile, by providing a policy file that describes them.