engine service add user_creds_id to create_stack interface

Adds the option to pass user_creds_id to create_stack in the service.py
so we can pass the parent user_creds reference in to nested stacks when
creating them via RPC, for compatibility with the way nested stacks
are currently created.

Change-Id: I127628d9af4c148ad7eaf6f0fc8cdb754cc9d6fe
blueprint: decouple-nested
Merge "engine service add user_creds_id to create_stack interface"