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 merging https://github.com/rackerlabs/otter/pull/466 and 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
    2013-11-19_05:45:41.41019
                {
                  "is_error": false, 
                  "event_type": "convergence.scale_up", 
                  "level": 6, 
                  "tenant_id": "851153", 
                  "_message": "Starting 1 new servers to satisfy desired capacity", 
                  "convergence_delta": 1, 
                  "scaling_group_id": "0e3cdf03-4263-47b7-8541-dc0dff40d0db", 
                  "desired_capacity": 1, 
                  "version": "1.0", 
                  "timestamp": 1384839941.410106, 
                  "pending_capacity": 1, 
                  "audit_log": true
                }

- from force-deleting the previous group
    2013-11-19_05:46:05.15274
                {
                  "is_error": false, 
                  "event_type": "convergence.scale_down", 
                  "level": 6, 
                  "tenant_id": "851153", 
                  "_message": "Deleting 1 servers to satisfy desired capacity", 
                  "convergence_delta": -1, 
                  "scaling_group_id": "0e3cdf03-4263-47b7-8541-dc0dff40d0db", 
                  "desired_capacity": 0, 
                  "version": "1.0", 
                  "timestamp": 1384839965.152643, 
                  "pending_capacity": 0, 
                  "audit_log": true
                }

- from executing a scale up after creating a group
    2013-11-19_05:48:12.64187
                {
                  "is_error": false, 
                  "event_type": "convergence.scale_up", 
                  "level": 6, 
                  "tenant_id": "851153", 
                  "_message": "Starting 1 new servers to satisfy desired capacity", 
                  "convergence_delta": 1, 
                  "scaling_group_id": "46937273-cca5-41f8-a7aa-351258d22785", 
                  "desired_capacity": 1, 
                  "version": "1.0", 
                  "timestamp": 1384840092.641651, 
                  "pending_capacity": 1, 
                  "policy_id": "2baa52b0-4408-48e9-b5a4-adadab048c6b", 
                  "audit_log": true
                }

Todo:
- [] remove policy id from the policy execution convergence messages