Add nested_depth to internal _create_stack RPC interface

Adds a nested_depth parameter, which can be used to set the correct
nested_depth value when creating nested stack via RPC calls between
engines.

Change-Id: I71538a4297168a08a1aaaf3d23f42f3878f6cf0e
blueprint: decouple-nested
Merge "Add nested_depth to internal _create_stack RPC interface"