Add owner_id to new _create_stack RPC interface

Adds a new "internal" _create_stack RPC call, which exposes a superset
of the existing RPC interfaces, intended only for engine-to-engine
usage, not for exposure via the user-facing API's (hence the prefix)

This patch exposes "owner_id" via the new call, which enables creation
of nested stacks via an RPC call, instead of creating the stack directly.

Change-Id: I355c608e657cb9dae2d2b38658b0b247040de6b9
blueprint: decouple-nested
Merge "Add owner_id to new _create_stack RPC interface"