Finer access control in os-volume_attachments

Allows policy.json access controls to authorize requests for specific
actions, rather than just being an all or nothing control for the entire
extension.

Bug 1108222
DocImpact
Change-Id: I78e1f596f22434a73bec3952ed024e4d58faac51
Merge "Finer access control in os-volume_attachments"