Audit log successful creation and successful deletion of servers (ready for review)

This was the easy one.  Still working on the failures.

This is the audit log for scaling up by 1, then down by 1:

    2013-12-05_00:36:25.97854
                {
                  "is_error": false, 
                  "event_type": "convergence.scale_up", 
                  "tenant_id": "851153", 
                  "@timestamp": 1386203785.978461, 
                  "_message": "Starting 1 new servers to satisfy desired capacity", 
                  "convergence_delta": 1, 
                  "scaling_group_id": "c640d3f9-e67a-4bb4-98d1-32c3b7772c1f", 
                  "desired_capacity": 1, 
                  "host": "ubuntu", 
                  "current_capacity": 0, 
                  "pending_capacity": 1, 
                  "@version": 1, 
                  "transaction_id": "36e44101-10f6-4d2d-9115-c1a9d0d65483", 
                  "audit_log": true
                }
    2013-12-05_00:36:26.04168
                {
                  "server_id": "test-server4434096851-id-4434096851", 
                  "is_error": false, 
                  "event_type": "server.active", 
                  "tenant_id": "851153", 
                  "@timestamp": 1386203786.041568, 
                  "_message": "Server is active.", 
                  "scaling_group_id": "c640d3f9-e67a-4bb4-98d1-32c3b7772c1f", 
                  "host": "ubuntu", 
                  "@version": 1, 
                  "transaction_id": "36e44101-10f6-4d2d-9115-c1a9d0d65483", 
                  "audit_log": true
                }
    2013-12-05_00:37:32.00438
                {
                  "is_error": false, 
                  "event_type": "convergence.scale_down", 
                  "tenant_id": "851153", 
                  "@timestamp": 1386203852.004307, 
                  "_message": "Deleting 1 servers to satisfy desired capacity", 
                  "convergence_delta": -1, 
                  "scaling_group_id": "c640d3f9-e67a-4bb4-98d1-32c3b7772c1f", 
                  "desired_capacity": 0, 
                  "host": "ubuntu", 
                  "current_capacity": 0, 
                  "pending_capacity": 0, 
                  "@version": 1, 
                  "transaction_id": "08e5e482-1ca8-46de-a974-c9b7dc8201d8", 
                  "audit_log": true
                }
    2013-12-05_00:37:32.03681
                {
                  "server_id": "test-server4434096851-id-4434096851", 
                  "is_error": false, 
                  "event_type": "server.delete", 
                  "tenant_id": "851153", 
                  "@timestamp": 1386203852.036702, 
                  "_message": "Server deleted.", 
                  "scaling_group_id": "c640d3f9-e67a-4bb4-98d1-32c3b7772c1f", 
                  "host": "ubuntu", 
                  "@version": 1, 
                  "transaction_id": "08e5e482-1ca8-46de-a974-c9b7dc8201d8", 
                  "audit_log": true
                }

This is the audit log for scaling up by 1, then force deleting the group before the server has built:

    2013-12-05_00:46:46.83932
                {
                  "is_error": false, 
                  "event_type": "convergence.scale_up", 
                  "tenant_id": "851153", 
                  "@timestamp": 1386204406.839227, 
                  "_message": "Starting 1 new servers to satisfy desired capacity", 
                  "convergence_delta": 1, 
                  "scaling_group_id": "e63b3140-87e1-4b15-a10c-0aac91e2caec", 
                  "desired_capacity": 1, 
                  "host": "ubuntu", 
                  "current_capacity": 0, 
                  "pending_capacity": 1, 
                  "@version": 1, 
                  "transaction_id": "af35b075-3cf0-4eab-9c17-b51d7f3d766f", 
                  "audit_log": true
                }
    2013-12-05_00:47:06.71649
                {
                  "is_error": false, 
                  "event_type": "convergence.scale_down", 
                  "tenant_id": "851153", 
                  "@timestamp": 1386204426.716414, 
                  "_message": "Deleting 1 servers to satisfy desired capacity", 
                  "convergence_delta": -1, 
                  "scaling_group_id": "e63b3140-87e1-4b15-a10c-0aac91e2caec", 
                  "desired_capacity": 0, 
                  "host": "ubuntu", 
                  "current_capacity": 0, 
                  "pending_capacity": 0, 
                  "@version": 1, 
                  "transaction_id": "e7d6eeb4-6e92-4fd9-8d71-2f7d3ea53c6f", 
                  "audit_log": true
                }
    2013-12-05_00:48:48.22277
                {
                  "server_id": "c33ca162-5856-4589-b550-320ecb0434f8", 
                  "is_error": false, 
                  "event_type": "server.deletable", 
                  "tenant_id": "851153", 
                  "@timestamp": 1386204528.222704, 
                  "_message": "A pending server that is no longer needed is now active, and hence deletable.  Deletion starting.", 
                  "scaling_group_id": "e63b3140-87e1-4b15-a10c-0aac91e2caec", 
                  "host": "ubuntu", 
                  "@version": 1, 
                  "transaction_id": "af35b075-3cf0-4eab-9c17-b51d7f3d766f", 
                  "audit_log": true
                }
    2013-12-05_00:49:01.14346
                {
                  "server_id": "c33ca162-5856-4589-b550-320ecb0434f8", 
                  "is_error": false, 
                  "event_type": "server.delete", 
                  "tenant_id": "851153", 
                  "@timestamp": 1386204541.143353, 
                  "_message": "Server deleted.", 
                  "scaling_group_id": "e63b3140-87e1-4b15-a10c-0aac91e2caec", 
                  "host": "ubuntu", 
                  "@version": 1, 
                  "transaction_id": "af35b075-3cf0-4eab-9c17-b51d7f3d766f", 
                  "audit_log": true
                }

I wasn't entirely sure whether we should have the `server.deletable` + `server.delete` message for canceled pendings.  The reasons for this are:

1. the code is slightly simpler - log everything, rather than special case deletions as a result of a delete vs as a result of a pending cancel
2. if the deletion fails this would be a good way for users to be able to see which one failed to delete.

I'm not sure I feel that strongly though, so hoping for opinions.