eve/schema: add host

The "host" field is added to EVE events if the "sensor-name" field is
configured in suricata.yaml.

etc/schema.json

@@ -39,6 +39,11 @@
         "flow_id": {
             "type": "integer"
         },
+        "host": {
+            "$comment": "May change to sensor_name in the future, or become user configurable: https://redmine.openinfosecfoundation.org/issues/4919",
+            "description": "the sensor-name, if configured",
+            "type": "string"
+        },
         "icmp_code": {
             "type": "integer"
         },