Fix service_user creation

The os_user module in newer versions of ansible (2.8.5) rejects to
create users where the default project is not existing in the user's
domain. So create the magnum_trustee_domain_admin user with an empty
default project.

For details see: https://github.com/ansible/ansible/pull/59876

Change-Id: I8e005df0de39bf2f4790c06ed1b311ff0caa070a

tasks/magnum_service_setup.yml

@@ -56,7 +56,8 @@
           {
             'name': magnum_trustee_domain_admin_name,
             'password': magnum_trustee_password,
-            'domain': magnum_trustee_domain_name
+            'domain': magnum_trustee_domain_name,
+            'project': ''
           }
         ]
       %}