magnum domain register error at setting domain id

Setting up Magnum fails at creating and setting magnum's trustee
domain.
The changes this patch introduces follow.
  - add registering trustee_domain
  - remove unnecessary set_fact as this variable is used only once

Change-Id: I1c1ed8a20871fb4e9f153c6dd1431d9b6fd286a5
Closes-Bug: #1667209

ansible/roles/magnum/tasks/register.yml

@@ -42,12 +42,9 @@
       auth: "{{ '{{ openstack_magnum_auth }}' }}"
     module_extra_vars:
       openstack_magnum_auth: "{{ openstack_magnum_auth }}"
+  register: trustee_domain
   run_once: True
 
-- name: Setting Magnum trustee domain value
-  set_fact:
-    magnum_trustee_domain_id: "{{ trustee_domain.id }}"
-
 - name: Creating Magnum trustee user
   kolla_toolbox:
     module_name: "os_user"
@@ -64,7 +61,7 @@
   kolla_toolbox:
     module_name: "os_user_role"
     module_args:
-      domain: "{{ magnum_trustee_domain_id }}"
+      domain: "{{ trustee_domain.id }}"
       user: "{{ magnum_trustee_domain_admin }}"
       role: "admin"
       auth: "{{ '{{ openstack_magnum_auth }}' }}"