Add volume RPC API v2.0
This patch creates volume RPC API version 2.0, while retaining
compatibility in rpcapi and manager for 1.x, allowing for continuous
deployment scenarios.
This should be merged just before the Mitaka release.
UpgradeImpact - Deployments doing continous deployment should not
upgrade into Newton before doing an upgrade which includes all the
Mitaka's RPC API version bump commits (scheduler, volume, backup).
Change-Id: Ic7df6fdc1ce8ab0b3782161a6b6769f45883dba6
Related-Blueprint: rpc-object-compatibility