Add instance.share_attach notification

This patch add a notification when a share is attached to an instance.

Manila is the OpenStack Shared Filesystems service.
These series of patches implement changes required in Nova to allow the shares
provided by Manila to be associated with and attached to instances using
virtiofs.

Implements: blueprint libvirt-virtiofs-attach-manila-shares

Change-Id: I6fe3807164bd2ca6439342abd32f8e4ce9057c8c

doc/notification_samples/common_payloads/InstanceActionSharePayload.json

@@ -0,0 +1,9 @@
+{
+    "$ref": "InstanceActionPayload.json",
+    "nova_object.data":{
+        "share_id": "e8debdc0-447a-4376-a10a-4cd9122d7986"
+    },
+    "nova_object.name": "InstanceActionSharePayload",
+    "nova_object.namespace": "nova",
+    "nova_object.version": "1.0"
+}

doc/notification_samples/instance-share_attach-end.json

@@ -0,0 +1,8 @@
+{
+    "event_type": "instance.share_attach.end",
+    "payload": {
+        "$ref": "common_payloads/InstanceActionSharePayload.json#"
+    },
+    "priority": "INFO",
+    "publisher_id": "nova-api:compute"
+}

doc/notification_samples/instance-share_attach-start.json

@@ -0,0 +1,8 @@
+{
+    "event_type": "instance.share_attach.start",
+    "payload": {
+        "$ref": "common_payloads/InstanceActionSharePayload.json#"
+    },
+    "priority": "INFO",
+    "publisher_id": "nova-api:compute"
+}