Create stack_domain_admin user

Create an additional service user for Heat, which is a domain admin
for the stack_user_domain - this is necessary since the normal service
user cannot manage the projects/users in the stack_user_domain when
keystone is configured to use the v3cloudsample policy (such as in
gate integration tests).

Change-Id: If59c11a74145b9bd02f78a7e0882afe1b0a72e40
Merge "Create stack_domain_admin user"