Define matrix-corporal's systemd service manager priority in terms of `matrix_homeserver_systemd_service_manager_priority`

Related to 4761ff7e9a

group_vars/matrix_servers

@@ -613,7 +613,7 @@ devture_systemd_service_manager_services_list_auto: |
     +
     ([{
       'name': 'matrix-corporal.service',
-      'priority': 1500,
+      'priority': (matrix_homeserver_systemd_service_manager_priority + 500),
       'restart_necessary': (matrix_corporal_restart_necessary | bool),
       'groups': ['matrix', 'corporal'],
     }] if matrix_corporal_enabled else [])