engine service add stack_user_project_id to create_stack interface

Adds the option to pass stack_user_project_id to align with the recent
change which makes nested stacks inherit the stack user project from
their parent ref bug #1343163.  This will enable the same behavior to
be maintained when creating stacks via RPC calls instead of directly.

Change-Id: I2a102722d3e5228ccbb71ca13f4246725626f5a9
blueprint: decouple-nested
Merge "engine service add stack_user_project_id to create_stack interface"