Add support for policy.d per service

[Mathieu Gagné]

The oslo.policy library allows services to load multiple policy files from multiple policy_dirs, usually policy.d. Some projects such as Neutron stadium projects also make use of policy.d to install and use additional policy files.

Current implementation of django-openstack-auth does not support more than one policy file per service. This means it is impossible for an operator to use an unified set of policy files (and directories) between services and django-openstack-auth as the later expects one single big policy file.

This bug report suggests adding support for policy_dirs per service so an operator can load multiple policy files just like they can with services.

[Mathieu Gagné (mgagne)]

Closing in favor of blueprint: https://blueprints.launchpad.net/django-openstack-auth/+spec/policy-dirs