Add stack_user_project_id to _create_stack RPC interface

Adds the ability to pass the parent stack_user_project_id into nested stacks
when creating them via RPC, so we can maintain the recently introduced
behavior where the parent stack user project is inherited by nested
stacks.

Change-Id: I425e80cfc8f9c646ade7d47f202744103142ab55
blueprint: decouple-nested
Merge "Add stack_user_project_id to _create_stack RPC interface"