Log convergence events up (ready for review)

Upon `obey_config_change` and `maybe_execute_scaling_policy`

This isn't exactly neat.  In fact, the convergence log may unfortunately show up a couple milliseconds before the policy execution/config change/force delete request log.  Not sure yet (haven't gotten to the request logs - figured the convergence log was more important.

But we'll hopefully be moving to a for real convergence model soon.

By using the script from https://github.com/rackerlabs/otter/pull/462, I got the following:

- from creating a group with a min-entity of 1

                {
                  "is_error": false, 
                  "event_type": "convergence.scale_up", 
                  "tenant_id": "851153", 
                  "@timestamp": 1384895385.807099, 
                  "_message": "Starting 1 new servers to satisfy desired capacity", 
                  "convergence_delta": 1, 
                  "scaling_group_id": "c4f1906b-27c9-414f-ae34-4a4efa64548d", 
                  "desired_capacity": 1, 
                  "pending_capacity": 1, 
                  "@version": 1, 
                  "audit_log": true
                }

- from force-deleting the previous group

                {
                  "is_error": false, 
                  "event_type": "convergence.scale_up", 
                  "tenant_id": "851153", 
                  "@timestamp": 1384895589.943218, 
                  "_message": "Starting 1 new servers to satisfy desired capacity", 
                  "convergence_delta": 1, 
                  "scaling_group_id": "f6a4974c-7d2b-4cd0-b0f3-29b72d6499ea", 
                  "desired_capacity": 1, 
                  "pending_capacity": 1, 
                  "@version": 1, 
                  "audit_log": true
                }
- from executing a scale up after creating a group

                {
                  "is_error": false, 
                  "event_type": "convergence.scale_down", 
                  "tenant_id": "851153", 
                  "@timestamp": 1384895618.179711, 
                  "_message": "Deleting 1 servers to satisfy desired capacity", 
                  "convergence_delta": -1, 
                  "scaling_group_id": "f6a4974c-7d2b-4cd0-b0f3-29b72d6499ea", 
                  "desired_capacity": 0, 
                  "pending_capacity": 0, 
                  "@version": 1, 
                  "audit_log": true
                }

Caveat - currently we do not generate a new transaction id for the convergence.  Not sure we should until it's actually a separate transaction.