Implement secure RBAC for share network subnets
Implement secure RBAC for share network subnets

This commit updates the policies for share network subnets 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: I2c90a40a7950be0463c3bc1bcf0b2d41cb6aeaa7

openstack/manila,openstack/manila