Converge on desired load balancer state (no draining)

This is a first stab at converging on desired load balancer state.  This PR adds the following functionality:

1.  Removes a server from all the load balancers it's attached to when the server needs to be deleted
2.  Converges all active servers to their desired load balancer state

Remaining to be done):
- [x] Tests for _converge_lb_state
- [x] Function to create the current LB status structure in the `NovaServer` class from `load_balancers_with_cheese`, plus tests.  (Also maybe should be in different PR)
- [x] Tests for converge to handle LB removal

Points I'm unsure about that would be awesome if addresesd in review:

1.  Naming
2.  We do not currently support specifying the condition or type or weight of the server on a LB.  Weight makes sense - I wonder if it would ever make sense to support the SECONDARY type in an autoscale group.  (traffic is only sent to a SECONDARY node if one of the PRIMARY nodes goes down).

Thanks!

UPDATE: 
In future PR:  function for aggregating all AddToLoadBalancer calls for the same load balancer into one step.  Same for RemoveFromLoadBalancer.

Oops, durr. I forgot that this was an interface you were introducing, so you get to pick what it means.

I don't think there's anything wrong with the way it is; I just wanted to know when it is falsy (and apparently the answer is "when the user does not specify a ServiceNet"). It's a bit unfortunate that you can't really specify "str or None" or "nullable str". Another case for arbitrary predicates, I guess?

Meanwhile, I think the empty string default is fine :)