Add BDM to InstancePayload

This patch adds bdm related data to InstancePayload. As loading BDM
can be expensive a new configuration flag 'bdms_in_notifications' is
introduced. If this is set to false (as by default) then the instance
notifications will contain 'null' value in block_devices field.
If this configuration flag is set to true then the block devices will
be loaded from the db and will be included in the notification payload.

Implements: bp additional-notification-fields-for-searchlight
Change-Id: I18e7483ec9a484a660e1d306fdc0986e1d5f952b

doc/notification_samples/instance-rebuild-end.json

@@ -3,7 +3,7 @@
     "publisher_id": "nova-compute:compute",
     "payload": {
         "nova_object.namespace": "nova",
-        "nova_object.version":"1.3",
+        "nova_object.version": "1.4",
         "nova_object.data": {
             "node": "fake-mini",
             "fault": null,
@@ -65,6 +65,18 @@
             "uuid": "b271fcb9-75c3-4c76-84eb-6ccad1150ece",
             "locked": false,
             "availability_zone": "nova",
+            "block_devices": [{
+                "nova_object.data": {
+                    "boot_index": null,
+                    "delete_on_termination": false,
+                    "device_name": "/dev/sdb",
+                    "tag": null,
+                    "volume_id": "a07f71dc-8151-4e7d-a0cc-cd24a3f11113"
+                },
+                "nova_object.name": "BlockDevicePayload",
+                "nova_object.namespace": "nova",
+                "nova_object.version": "1.0"
+            }],
             "ramdisk_id": "",
             "architecture": null,
             "progress": 0,