Make volume_snapshot_{create,delete} use objects

This patch makes the volume_snapshot_create and delete methods use
new-world Instance objects.

It bumps the version of the compute rpc API, and starts passing objects
to the rpc layer. It also makes the driver methods that touch instance
use the dot notation to access the attributes.

It also makes sure that the backwards compatibility is kept by
decorating the manager methods with object_compat decorator.

Part of the blueprint: icehouse-objects
Part of the blueprint: clean-up-legacy-block-device-mapping

Change-Id: I967061cef36b4da06073736fce871fc3f3d28f97
Merge "Make volume_snapshot_{create,delete} use objects"