heat_keystoneclient add delete_stack_domain_user function

Add function which deletes a user in the heat domain user domain,
in the project specified.  Since we're deleting via the admin client
add some sanity checks to ensure we're deleting a user in the
correct domain/project.

Change-Id: I6a29164147db0d660e6a0cab9b27ea0fc55d99ba
blueprint: instance-users
Merge "heat_keystoneclient add delete_stack_domain_user function"