Volume sharing between instances

This blueprint adds support for shared volumes without using Cinder's proposed extensions mechanism. Shared volumes allows attaching the same volume, under either RW or RO mode, to multiple instances. Cinder in this case only manages the volume attach consistency, the file system consistency issues are in the domain of the applications running on the instances. Volume sharing is restricted to the instances within a tenant, the objective is not to provide cross tenant volume access. The shared volumes functionality allows for the creation and deletion of volumes that are shareable; attaching and detaching (with nova volume-attach/detach) shareable volumes to multiple instances and managing shared volumes in dashboard.