Add user_creds_id to _create_stack RPC interface

Adds the ability to pass the parent user_creds_id into nested stacks
when creating them via RPC, so we can maintain the current behavior
in StackResource, where the parent user_creds record is used by all
nested stacks.

Change-Id: Ifed93860db68a7d23b0c05bd9ec83619e109e386
blueprint: decouple-nested
Merge "Add user_creds_id to _create_stack RPC interface"