(dreid, alex) Fixed a bug with adding a server to multiple load balancers.

The callback that was added formed a closure over information about the load
balancer however due to the way binding of names in closed scopes works in
Python, every callback would receive the value from the last load balancer.
Add an extra line break for clarity and use lb_id for consistency.
Merge pull request #225 from rackerlabs/fix-multiple-load-balancers

(dreid, alex) Fixed a bug with adding a server to multiple load balancers.