Implement secure RBAC for share snapshot instances
Implement secure RBAC for share snapshot instances

This commit updates the policies for share snapshot instances 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: I6ec289e82d8f37ea7e832476345a3cac42662280

openstack/manila,openstack/manila