heat_keystoneclient add create_stack_domain_user function

Add function which creates users in the heat domain user domain,
in the project specified (which is expected to be in the correct
domain, ie created via create_stack_domain_project)

This is similar to create_stack_user, which is expected to be removed
when all resources have been migrated to domain users.

Change-Id: Ie0a6ce039058a574ff0ddc3ed69e0e9cc9ad4db4
blueprint: instance-users
Merge "heat_keystoneclient add create_stack_domain_user function"