engine service add nested_depth to create_stack

Adds nested_depth as an optional argument to the top-level interface
for handling stack-creates, so it will be possible to specify the
depth when creating a nested stack via an RPC call between engines.

Change-Id: Idc24d1e55810b10615d764b94f7c708673516295
blueprint: decouple-nested
Merge "engine service add nested_depth to create_stack"