Convert heat_keystoneclient user-create logic to v3 API

Migrate the calls to create the stack user and assign the
stack user role to the v3 API (still using the default domain)

Change-Id: I34d07cd6c61aa7fccbd386f01504cb18a1f09803
blueprint: keystone-v3-only
Merge "Convert heat_keystoneclient user-create logic to v3 API"