Migration history extenstion

Add a extension to list migration history for instances.

Admins are frequently in need of reviewing migration history for instances for various
purposes. There is currently no clean way to do this. This purposes to add an extension
to access this information through the openstack API, so that we dont have to go to the
db to get this info.

Usage:
GET on
/v2/<project_id>/servers/<server_uuid>/os-migration-history

to get migration history for the instance <server_uuid> in the form:
{
     "migrations": [
         {
             "created_at": "2013-03-25 01:00:00.000000",
             "dest_compute": "compute-2",
             "id": "1",
             "instance_uuid": "00000000-0000-0000-0000-000000000001",
             "new_instance_type_id": "2",
             "old_instance_type_id": "1",
             "source_compute": "compute-1",
             "status": "confirmed",
             "updated_at": "2013-03-25 01:20:00.000000"
         }
     ]
 }