Comment 10 for bug 1567471

Scenario checked: get api request GET http://10.109.5.2:8000/api/v1/graphs
"model": "cluster" relations are deleted when clusted is deleted.
Verified on:
cat /etc/fuel_build_id:
 443
cat /etc/fuel_build_number:
 443
cat /etc/fuel_release:
 9.0
cat /etc/fuel_openstack_version:
 mitaka-9.0