Implement secure RBAC for agents API
Implement secure RBAC for agents API

This commit updates the policies for agents API to understand scope
checking and account for a read-only role.

This is part of a broader series of changes across OpenStack to
provide a consistent RBAC experience and improve security.

Partially-Implements blueprint: secure-rbac-roles

Change-Id: I28972b2c07d406b2719726e1713ce81be915f5a7

mahak/neutron,openstack/neutron,mahak/neutron,openstack/neutron,mahak/neutron,openstack/neutron