This change introduces the two RPC calls required for the allocation
API: create_allocation and destroy_allocation.
The nodes RPC is updated to:
* Prevent instance_uuid deletion if a node has an allocation and is
not in an updatable state.
* Delete allocation when instance_uuid is deleted and the node is
in an updatable state.
* Delete allocation when a node is unprovisioned and instance_uuid
is thus cleared.
Change-Id: I45815727f970c3d7fe51bb78d8e162a374d12e04
Story: #2004341
Task: #27987