Change attribute on bp/abort-live-migration

On api-wg guideline[1], we recommend to use "server" on REST APIs
instead of "instance" like

 server vs. instance
   server shall be used to describe a virtual machine, a bare-metal
   machine, or a containerized virtual machine that is used by OpenStack
   users for compute purposes. The older term instance that is also by
   Amazon Web Services EC2 API to describe a virtual machine, should not
   be used in OpenStack REST APIs.

So this patch changes the attribute which contains "instance" for fitting
to the guideline.

Partially blueprint abort-live-migration

[1]: https://github.com/openstack/api-wg/blob/master/guidelines/terms.rst

Change-Id: I5c0fc31f958278d8052aa58e19c99cb4b800a8d3

specs/mitaka/approved/abort-live-migration.rst

@@ -190,7 +190,7 @@ Body::
         "created_at": "2013-10-29T13:42:02.000000",
         "dest_compute": "compute3",
         "id": 6789,
-        "instance_uuid": "instance_id_123",
+        "server_uuid": "6ff1c9bf-09f7-4ce3-a56f-fb46745f3770",
         "new_flavor_id": 2,
         "old_flavor_id": 1,
         "source_compute": "compute2",