Omit passing the project when no project scope is needed

With recent changes to role_assignment module, passing an empty
`project` is treated as an error, as module attempts to set
project scope even when project is null or is empty.

In order to overcome this we apply extra logic to preserve
current behavior while allowing to avoid passing project
when the value is passed as an empty ones by roles, like Heat[1]

[1] 8dd53ea2f1/tasks/heat_service_setup.yml (L61)

Closes-Bug: #2103512
Change-Id: Ic088dfd7aa0958d66d4fa2b10d8e72496cbb7a71

roles/service_setup/tasks/setup_roles.yml

@@ -36,12 +36,13 @@
 - name: Add service users to roles
   vars:
     role_mapping: "{{ (role is string) | ternary({'name': role, 'state': 'present'}, role) }}"
+    user_project: "{{ user.project | default(_service_project_name) }}"
   openstack.cloud.role_assignment:
     cloud: "{{ _service_cloud_name }}"
     state: "{{ role_mapping['state'] | default('present') }}"
     user: "{{ user.name }}"
     role: "{{ role_mapping['name'] }}"
-    project: "{{ user.project | default(_service_project_name) }}"
+    project: "{{ (user_project is truthy) | ternary(user_project, omit) }}"
     domain: "{{ user.domain | default('default') }}"
     endpoint_type: admin
     validate_certs: "{{ not (_service_adminuri_insecure | default(True) | bool) }}"