Alarm on a non-existing resource adds the resource to the graph

[Ifat Afek]

Alarm on a non-existing resource causes adding the non-existing resource to the graph.

To reproduce:
- enable the Doctor datasource (add 'doctor' to 'types' in /etc/vitrage/vitrage.conf)
- send a 'compute.host.down' event that is translated by the Doctor datasource to an alarm on a host. Set a non existing hostname.

vitrage event post --type="compute.host.down" --details='{"hostname": "non-existing-host","source":"sample_monitor","cause":"link-down","severity":"critical","status":"down","monitor_id":"monitor-1","monitor_event_id":"123"}' --time='2017-07-03T15:02:01.00000'

Expected result:
- A compute.host.down alarm that is not connected to any host
- The non-existing host is marked as a placeholder and is not visible in the entity graph

Actual result:
- A compute.host.down alarm that is connected to a non existing host