Use load balancer configuration, not load balancer id, in job-based LB removal logic

This is required to support RCv3.
[`delete_server`](https://github.com/rackerlabs/otter/blob/master/otter/worker/launch_server_v1.py#L855) would also have to be changed to synthesize a dummy `lb_config` instead of just passing the load balancer ID.

Apologies, I think I missed some discussions.  What is the plan for RCv3?  Is it supposed to save a completely different `lb_info` blob for the load balancer data?  If so, how will `delete_server` handle it?