Implement secure RBAC for share instance export location
Implement secure RBAC for share instance export location

This commit updates the policies for share instance export locations 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.

Change-Id: I0cf9beed3c60fd790045580afa0c993c21e71d49

openstack/manila,openstack/manila